I thought auto-invasion success was fixed towards the end of the last run? Doesn't seem to be the case glancing through legends. Dwarves always seem to survive 250 years (albeit a bit battered) in the worlds I've tried. Always large worlds, with 'very high' sites.
Annoyed to discover my Fury population is 6(!) in my new world. Result of 233 year war with Hellfire Imps. Ruthertaurs didn't make it all, wiped out by roc and colossus (my fault for setting beasts at very high I guess).
In other news, I see Toady has fixed mounted goblins, which is good news for future versions of this mod.