Well, I don't remember all the details, but I remember agonizing over class choices in M&MX. My conclusion was that they are almost perfectly balanced and almost all very potent (well, except the Defender).
Each skill access level has a reason and is balanced against other options. Some things may surprise (like lack of GM access for a specific weapon, skill), but it all has been clearly very well though out and the class certainly has other perks. Mercenary from your example doesn't have a weapon GM, but has Warfare GM instead (only him?), which makes up for it.
None of the classes are weak, and you can complete the game with any of the classes, but not all of them are equally strong. There is nothing the Mercenary can do that others can't do better. He has GM Warfare which lets him get an automatic critical hit once every turn by spending mana on it, this is where he shines. The problem is that you're at late-mid to end game when you're able to pick up this ability, and by then he's dealing half of the damage that a Barbarian will do. Grandmaster more than doubles your damage, and this gets more noticeable when you pick up +5/+6 sword/axe/mace gear from the shops. The Mercenary is the ONLY might hero that can't get Grandmaster in a weapon.
I would rate the Defender above than the Mercenary, and he can get both Grandmaster Axe AND Grandmaster Warfare. The problem with the Defender is getting him a good weapon, once you solve that problem he's surprisingly solid. You'll want to build him 3 Might/1 Vitality on level up.
The last party I ran with was able to kill the Cyclops and Shadow Dragon at level 8 (always Warrior difficulty) and I went on to breeze through the game with this party, even one shotting the last bosses. Crusader, Defender, Barbarian and Druid. The Barbarian dealing 1200+ on a critical hit was wonderful to see.