After some lengthy testing, it appears that the single link to worldgen crashing is the number of sites. When the number of sites is low (per the amount of civs placed, e.g. only 420 sites at 60 civs), the crashes are uniformly 100%; when the number of sites is higher (e.g. 1440 sites at 45 civs; it seems that the effect diminishes from some point and even at no site cap the statistics appear roughly the same) then around 2 worldgens out of 7 are successfully finished without crashing. The history length doesn't seem to matter (or, at least, didn't in my tests ranging from 550 year to 1 year worldgens), nor do the specific entities seem to be at blame (I've tried narrowing it down to "culprits") - although at certain point increasing the number of entities alone seems to force the crashing worldgen behavior. Errorlog doesn't show anything at all, only the occasional "Impoverished Word Selector"
(Big thanks to Illgeo for initially spotting the connection between the sites number and successful worldgens!)
Seems to me that this might be a hidden vanilla bug.
Also, I've noticed that beastmen civilizations are almost always completely destroyed after just a hunderd years from the start; Illithids closely follow, but at least they often survive with just one or three civ members. I understand that this depends a lot on the specific geographical features (space to expand), favourable political placement, etc., but it seems a little too drastic - out of 40 or so successful worldgens (all past year 150), maybe 3 or 4 of them had beastmen civilizations almost completely dead (less than ten civ members), and just two worlds had them prolific (in the thousands in one case and tens of thousands in the other) - and in all other cases, their civilizations have been completely wiped for decades or centuries. Not even any middle variants like "in the hundreds" (quite common with orcs and centaurs, for example, among others).
Without overly changing the beastmen civilization, could there be a simple and sufficient fix to increase their survivability in worldgen?