in my pirate coast campaign Dark Elves and High Elves are pretty much in a constant stalemate all over South Lustria, both seem to be pretty strong.

I would say Dark Elves, High Elves and Lizardmen for having end game stacks that are extremely hard to defeat in 1v1 army combat, which will happen because legendary lords seems to always have lightning strike.

Though thanks to army caps, full carno/stego combo stacks or any of this ♥♥♥♥ is extremely rare. But as Human you can pull it off eventually in the late game.

Ikits skaven are extremely powerful, just as in the base game due to boatload of beneficial mechanics, army wiping nukes and Skaven being Skaven. Though he suffers from quite nightmarish starting position where everyone arounds you hate your guts, Wood Elves, Dwarves, Humans and Orcs from the right. The only factions that dont hate you from get go are Sartosa's pirates and Khemri (Large empire rep boost, assuming you manage to get more cities)