I recently read “Deconstructing ‘height dissimilation’ in Modern Greek” (Journal of Greek Linguistics 3, 2002) by Julián Méndez Dosuna. I don’t really dabble in Modern Greek dialectology, but this struck me as an interesting paper for its methodology regardless, and the lessons seem to apply also more widely.
The story goes: Modern Greek varieties often reflect Ancient Greek /ea eo/ as /ia io/, and while AGk /oa/ was more rare, it can be also reflected as ModGk /ua/.  This has traditionally been explained to have come about a process of height dissimilation: [mid] + [non-close] > [close] + [non-close]. JMD however argues for a different pathway. Using /ea/ for illustration, the first stage would rather have been coalescence to a diphthong /e̯a/, followed by unconditional raising of the nonclose nonsyllabic to give /ja/ — both reflexes also attested among the palette of ModGk dialects — and finally re-breaking to /ia/. His main objection is that mid vowel dissimilation seems to be phonetically unmotivated, that explaining it as a means to prevent syllable contraction is too teleological, and that this explanation makes no sense anyway for dissimilation feeding into glide formation (which is the traditional routing of varieties showing /ja/).
I am fully on board with this kind of an approach. It is my experience that dialectologists quite often (1) operate on an assumption of deriving modern dialects directly from a classical/standard variety of the language, and (2) do not have a good knowledge of comparative linguistics besides their own subject. Because of this they can end up proposing all kinds of historically backwards and/or phonologically nonsensical reconstructions or sound changes. Two examples from elsewhere would be alleged /q/ > /ɢ/, /g/ in Arabic dialects (surely rather an earlier split with something like (*kʼ >) *k̰ˤ > *q̰ > /q/ in Classical Arabic versus *k̰ˤ > *q̰ > /ɢ/ > /g/ or *k̰ˤ > *k̰ > /g/ dialectally)  and alleged conditional /aɪ aʊ/ > [əɪ əʊ] in Canadian English (surely rather Early Modern English *əɪ *əʊ being positionally retained and only conditionally lowered to /aɪ aʊ/).
If this alone wasn’t enough though, JMD covers also plenty of indirect reasons to prefer a glide formation + breaking pathway. From the Greek dialect data we have the following points:
- While mid + mid /eo/ can develop to /io/, the sequences /ee/ and /oo/  do not develop to **/ie/, **/uo/, and they instead generally show contraction to simple /e/, /o/).
- Glide formation explains concomitant stress retraction from e.g. /éa/ to /iá/ in some dialects, and also “regular hypercorrection” from e.g. /iá/ to /ía/ in others; or per JMD rather: stress advancement upon the re-breaking of /ja/ to /ia/.
- Re-breaking explains the history of dialects where e.g. /ia/ (from earlier /ea/ or not) appears to have given /ja/ only after “palatalizable” consonants, into which the glide is then absorbed; i.e. /nia/ > *ɲja > /ɲa/, but /ðia/ remains unchanged. Per JMD, the latter rather gives intermediate *ðja as well, but reverts to bisyllabic after *ɲj > /ɲ/ coalescence has applied.
- Also in varieties where the bisyllabic realization remains prescribed, sequences starting with a mid vowel can parse as a single syllable in poetry, and phonetic diphthongs such as [e̯a] can observed in connected speech.
As two additional typological arguments, he notes that mid vowel dissimilation, i.e. raising only before open or non-close vowels, is not well-attested as a synchronic phonological process, and that diphthongs do show a strong cross-linguistic tendency towards fully close endpoints. 
I didn’t catch this point being made particularly explicitly, but also linking /e̯a/ and /ja/ diachronically together additionally seems like increased economy over the traditional assumption of two unrelated coalescence processes along the lines of /e̯a/ < /ea/ > /ia/ > /ja/.
This all naturally makes me wonder about Finnish, where mid vowel dissimilation is a classic dialect feature, applying to unstressed /ea eä oa öä/ sequences. These primarily come about following elision of earlier unstressed *-ð- and are primarily found in four morpholexical environments: adjectives in -eA; partitive singulars in -A of nominal stems in -e-, -O-; infinitives in -A of verb stems in -e-, -O-;  “contracted” verbs in -A- derived from stems in -e-, -O-. All of these yield /ia iä ua yä/ in a variety of dialects, maybe best known as a feature of South Ostrobothnian, but also attested further north; in a small area in the southwest; and a slightly wider area in the southeast. 
Let’s first take a moment to consider if in Finnish, too, the /ia/ type reflexes could have actually followed the same /e.a/ > /e̯a/ > /ja/ > /i.a/ trajectory that JMD argues for modern Greek. Just as in Greek, the intermediates could be partly attested: /ja/ is known from a few southwestern and SOstrobothnian varieties, and some eastern varieties show /ea̯/, trivially close to more hypothetical *e̯a. (These are generally dialects that also show /oa̯/ and /eä̯/ for earlier unstressed *aa and *ää, and in principle one could propose that /eä oa/ actually first assimilate to *ää *aa; but for /ea̯ öä̯/ an explanation like this isn’t possible.) It is also the case that /jV/ > /iV/ under some particular conditions is a widely-distributed sound change in Finnish, e.g. /vjV/ > /viV/ in kavia ~ kavio ‘hoof’ < kavja ~ kavjo < ⁽*⁾kapja. I already think this might apply also in more cases than has usually been realized, and perhaps we could go further still and even assume developments such as korkea > korkja > korkia ‘tall’. Three-consonant clusters like /rkj/ would be rather strange to most Finnish dialects however.
There are also some adjective doublets that could be taken to suggest /eA/ > /iA/ > /jA/. Directly attested are at least eheä ~ ehjä ‘whole’, norea ~ norja ‘pliable’, sorea ~ sorja ‘beautiful’. Similar alternation can be reconstructed also behind at least lakea ~ laaja (< *laɣϳa < *lakja) ‘wide’ and välkeä (← *väleä by suffix exchange) ~ väljä ‘loose’. I am far from certain though about explaining these as phonological doublets. The variants in /-jA/ can be found also in dialects where the soundlawful development is /eA/ > /ee/, e.g. ehjä is found all across Tavastian dialects, and penetrates fairly well into Savonian dialects as well. In at least two cases this alternation even appears just within Karelian, where there is no sign of *eA > ˣ/iA/: kahei (Livvi) ~ kahja ‘coarse, rough’, karie ~ karja ‘coarse, big’. The latter indeed seems to be a specialization of Proto-Finnic *karja ‘cattle; multitude’, i.e. not a secondary development from a **kareda > **karea. My working hypothesis remains that this is mostly a kind of phonetically motivated morphological analogy, and that the forms in /-CjA/ are generally more original.
A final problem is that unlike Greek, Finnish has also original /-CjV/, as in *karja above. Some development to /-CiV/ can be found, but not in all cases. E.g. in SOstrobothnian /-ljV/, /-rjV/ > /-liV/, /-riV/ is regular, but /-hjV/ rather receives an echo vowel, e.g. pohja > pohoja ‘bottom; north’, tyhjä > tyhyjä ’empty’, clearly distinct from e.g. kauhea > kauhia ‘terrible’.
So a coalescence + re-breaking hypothesis runs into a variety of trouble in Finnish. I still would not want to just abandon the argument about vowel height dissimilation being an unnatural sound change though. Another way to fix the situation is possible too: glide epenthesis, followed by raising conditioned by this new glide (a mechanism that JMD passingly reports from Dutch). Thus, I would propose /ea eä oa öä/ > (? [ee̯a ee̯ä oo̯a öö̯ä] >) /eja ejä owa öɥä/ > /ija ijä uwa yɥä/ > /ia iä ua yä/. This has the same benefits of better typological plausibility, and no major problems with intermediate stages. The intermediate /eja ova/ type is again attested, conveniently neighboring both the SOstrobothnian and the southeastern /ia/ areas. Better still, there’s even the benefit that all three changes can be independently attested in Finnish!
- Glide epenthesis is a widely spread strategy of hiatus resolution in Finnish dialects, clearly especially in stressed syllables (where typically no further general changes apply); possibly also in unstressed syllables, i.e. in cases of the type *kataɣa >> kataja, SW katava ‘juniper’. These, too, might be at least partly epenthetic glides appended to *kata.a, rather than direct reflexes of *ɣ. (However, *-aða > *-a.a > /-aa/ appears to be exceptionless.)
- Raising of unstressed /e/ to /i/ before /j/ is well-attested all across Finnish, e.g. in actor nouns from e-stem verbs (sure- ‘to mourn’ → surija ‘mourner’). (No similar change applies with a labial glide, though: sanova ‘saying’ never gives ˣsanuva. Some eastern dialects show instead labial coloring, e.g. tuleva ‘coming’ > tulova; perhaps a more natural effect of the labiodental glide [ʋ].)
- Even today Finnish really shows no distinction between unstressed [ijV] and [i.V]: contrasts such as nauttia ‘to enjoy’ vs. nauttija ‘enjoyer’ are purely orthographic. Subphonemic variation between [u.V], [y.V] and [uwV], [yɥV] also appears, particularly conspicuous after stressed syllables (e.g. standard tauot ‘pauses’ is usually [tauwot ~ tawːot], not [tau.ot]).
This approach would also seem allow to explaining an interesting asymmetry in the small southwestern zone in Uusimaa, which shows only /ea/ >> /ia/ but no /oa/ >> /ua/ (rather /OA/ > /OO/). Here I would note that Finnish definitely has a phoneme /j/ anyway, but no /w ɥ/; maybe this resulted in /eA/ > *ejA but no epenthesis from /oa öä/ to **owa **öɥä. — A similar situation extends also to the southwestern dialects proper, which mostly show /ea/ >> /i/ but /oa/ >> /o/. The western Uusimaa dialects are already known for sharing also other features with SW Finnish, and to me it would seem the best to treat the former as an archaic sister group of the latter, not as an SW-influenced group of the Tavastian dialects (which do not form a single historical subgroup anyway). It seems that either *ia *oa or *ia *oo could be reconstructed as the typical pre-apocope reflexes in SW Finnish.
Altogether one very broad point this case study shows that while the phonological makeup of the Finnish dialects has been well-documented by now, the actual history leading up to them remains open to analysis.
 AGk /oe/, when not simply retained, gives however rather ModGk /oi/, or more exactly, the diphthong /oi̯/ = /oj/.
 An intermediate voiced stage for “*q” also explains why is Proto-Arabic *g spontaneously fronted to something like /ɟ/ or /dʒ/ in most varieties.
 I.e. bisyllabic [e.e], [o.o]; not to be confused with the AGk long vowels η ω /eː oː/ which I believe give short /i o/ in ModGk universally.
 I could quibble a bit with this last argument though. Certainly closing diphthongs such as /ai/, /au/ are ubiquitous, but it is not clear to me if close-to-open diphthongs like /i͡a/, /u͡a/ are actually substantially more common than mid-to-open diphthongs like /e͡a/, /o͡a/. But also variation between the two is common, and in all cases known to me, mid-to-open is moreover more archaic than close-to-open (thus e.g. Eastern Finnic, Western Mansi, Northern Samoyedic, several Samic varieties). This diachronic universal will be at least as good for the purposes of his argument, if not better, than JMD’s alleged synchronic universal.
 Verb stems in -e- are for some reason not covered by Kettunen’s dialectal atlas, perhaps since quite a few of them have instead consonant-stem infinitives, showing either assimilation of earlier *ð (pure- : purra ‘to bite’, tule- : tulla ‘to come’, mene- : mennä ‘to go’), late retention of *ð (näke- : nähdä ‘to see’), or blocking of lenition from *t to *ð to begin with (pese- : pestä ‘to wash’).
 The majority development, including modern colloquial Finnish and also most other Finnic varieties where deletion of medial *ð applies, is to instead contract these to long mid vowels /ee OO/, possibly followed by other changes such as diphthongization to /ie UO/ (thus e.g. Karelian proper) or shortening to /e o/ (thus e.g. Estonian).