In my Opinion, the biggest problem at time is, that the stats of the followers get reseted after each relog. That make them weak as hell. Thats the major bug atm, and it MUST be fixed bevore the are realy playable.
There are more minor bugs are out. Sometimes he dont return, sometimes he stuck or get lost, and realy rare he atacking himself until he has no mana/heal anymore. After u get him first time he do melee, and a ranged weapon dindt change that... only a relog helps.
And i think, some skills must be forbidden for followers. I even dindt try all skills, but i think, all work, and a merc, that sumons the whole time monsters, will be a little bit to strong, cause u have massive blockers.
The idea is great... Design ure own merc is great!