Oh yeah, I always forget that there were two groups, always lumped them together.
But honestly, out of the whole group, Leonardo is the only one whos bad. And thats only because Shinon outclasses him in every way imaginable. And well thats not surprising since Shinon is on par with Nephenee in terms of over-powered-God-tier.
Edward only loses to Mia in terms of resistance and speed, but trumps her in most everything else.
Nolan is easily one of the strongest units you can get in the beginning of the game. And unlike most starter units, can hold his own more then perfectly endgame.
And Sothe, hell anyone not using Sothe is just gimping themselves.
And then Micaiah is easily one of the most overpowered units end game. Takes a lot to build up, but is essentially nigh-unstoppable end game.
While her group takes more work and is not as powerful as the Mercenaries, they are not to be easily discounted. Other then Leonardo they are all pretty damn strong with the right amount of work. While they aren't as powerful as the Mercenaries can get, they aren't that much weaker, and you can beat the game with them without having to use the GM's.
But yeah, the Mercs are just insanely powerful from the moment you get them, and are insane until you beat the game. Micaiah's group can get almost as powerful, but takes WAAAAY more patience to build up.