Questions arise only because they removed the tank of two factions and the barbarian (melee DPS) of two factions... while leaving other factions with both tanks and barbarians. Jacobs has said they don't expect a balance issue, despite the uneven role distribution. I think whether or not they're right depends on the fundamental nature of classes in Warhammer Online.
It's possible for the removal of classes (on one side but not the other) to not upset balance... but only if the classes are designed more in the style of Diablo 2 than in the style of typical MMOs.
- In a typical MMO, each class is designed primarily to fulfill a specific role in group combat. The class's efficiency outside a group or outside that specific role in the group is secondary and generally suffers in some way(s).
- In Diablo 2, on the other hand, each class is designed to be a damage-dealing powerhouse in itself, and any class can be tailored to a variety of roles in group combat. The specific skills, open skill selection and upgrade system, open stat selection, and mostly-open item system make this possible. Even the barbarian can be made into a ranged support role through skill and stat selection.
I know that WAR is abandoning many ridiculous MMO traditions. And I know they want every class to be capable of soloing. So it's possible that Jacobs is right, and there's really not a severe balance issue caused by this recent move by Mythic. We'll just have to wait and see, right?
In any case, I recommend Diablo 2 as a model for anyone working on a skill or class system.