A question from an email discussion, the answers to which I think would be interesting to others as well:
Are certain branches more valuable than others when it comes to their relevance to Uralic historical linguistics?
I cannot offer any kind of rigorous rankings; only my own impressions, and they will not be the most detailed or best-researched. But they will be something I hope.
1. Vowel phonology
To this day based primarily on Finnic, Samic and northern Samoyedic — which are also the languages that best preserve unstressed vowels and bisyllabic root structure, giving an additional good reason to think they might be more archaic than the others. Mordvinic fits well in with F&S, Mari is messier, Permic and Khanty are huge messes (but still far from terra incognita). Hungarian and Mansi behave reasonably well again IMO, but this is really a bit hard to see from published research when almost everyone insists on comparing them with Khanty in the first place and not with the rest of Uralic. Southern Samoyedic is quite simply understudied, usually treated as just an appendix to Northern.
2. Consonant phonology
This has a relatively even basis, the rough details for every language have been known already since the late 1910s. I guess Permic has retained the most phoneme-wise distinctions, and Finnic + Samic followed by Samoyedic and Mordvinic are the most important for reconstructing consonant clusters, but every language matters for something.
3. Inflectional morphology
Also a relatively even playing field with the rough details well-known. Samic and Samoyedic could be said to stand out somewhat for having fairly archaic possessive suffix and case systems. Hungarian has almost completely upturned its noun inflection and Permic is not too far behind, but even in these the verbs retain a typical enough Uralic shape.
4. Derivational morphology
There have been some general overviews in the past, but this is a topic that needs more work all around. Word derivation has been described well only for Finnic and Hungarian (individually; not for the two of them in comparison, and not even for Proto-Finnic). Tundra Nenets clearly has the third-best coverage thanks to Tapani Salminen’s A Morphological Dictionary of Tundra Nenets, but then to my knowledge this has not been worked into any kind of a historic framework so far. I have the impression there’s some good literature in Russian at least on Erzya and Komi? No idea how much of a historical angle they would have.
No contest here. Finnish is surely the lexically best documented language in the world, with the Dictionary of Finnish Dialects archives covering 8.5 million records across perhaps 350 000 lemmas (contrast with “only” 300 000 lemmas in the Oxford English Dictionary, or 570 000 in English Wiktionary, despite orders of magnitude more speakers). Lexical documentation on Finnic in more widely is mostly in a pretty strong shape too, and so is etymology within the family. (For decades now most progress has come from loanword research though.)
Samic and Khanty are the next-most important. Huge dialect dictionaries have been available for a long time, and there is also a lexical reconstruction of Proto-Samic as well as an etymological dictionary of Khanty. Mansi could maybe eventually join this club if someone were to assemble a single comparative resource from all the individual ones. Komi and Udmurt are also similar but less diverse. Mari and Hungarian have been well documented, the latter also researched, but they are frankly not very rewarding due to lots of loanwords. Mordvinic falls between these and Permic. Samoyedic is documented quite heterogeneously, and also all reasonably large dictionaries other than for Tundra Nenets are very recent, there is surely going to be a lot of fodder for Uralic etymological research there still.
There are some good areal overviews, but then even the theory of syntactic reconstruction is not very advanced. (Towards this goal, I believe that a lot of internal reconstruction work has been accomplished that is kind of “hiding” in generative syntax, and is just waiting to be rewritten in actual historical terms… but this is generally almost all on languages other than Uralic.)
7. Anything else?
There would be some other but less strictly linguistic approaches to “historical Uralistics”, e.g. poetry, mythology, other ethnography; genetics and archeology even. This all falls outside my expertise however. The most I can say is that all this was a big part of the Uralic / Finno-Ugric studies paradigm a hundred or so years ago. Today much less so, now that we know that language, culture and genes are not quite as tightly bound together as romantic nationalists once assumed. Of course I still warmly support following such neighboring disciplines and continuing to integrate their results with linguistics where applicable.
Altogether then we end up with the three most diverse branches Finnic, Samic and Samoyedic (esp. Nenets) represented the best in research; Hungarian and Mari represented noticeably poorly, unless I have missed entirely some kind of major sources.