It all depends on your style of gameplay. I can easily take on (in BFME1) a couple hard
Mordor armies as Rohan. In BFME2 the same sort of system is set up: all factions have
their strengths and weaknesses, but not necessarily causing one faction to be greater than
another. If you have a cavalry-based style of play, you'll find Men or Elves more suited
to your tastes, whereas Isengard would be ideal if you use a lot of siege, etc. The main
thing with the different factions is their units, buildings, heroes, and powers. In
BFME2, however, the factions are more similar and "cloned" than they are in BFME1, but
there isn't a distinct advantage in either just by being a specific faction.