Balance answer: Imagine an invisible, flying, fireball throwing healer that can control your mind.
Sure, and what about it? It's more powerful than some characters may be, and less than others? I mean, balance? Well, a fully late armoured fighter with a magical weapon and a shield and "unbalanced" in a party made of unarmed farmers. Would you then create rules to forbid the existence in your world of armours and weapons since there would be unarmoured and unarmed people? After all, it would be "unbalanced.
I said it
countless of times, and
again but:
Nothing is truly simply "balanced" or "unbalanced" in the world, least in a game system. The answer depends on what the other players are playing, what the levels of their characters are, in comparison to the other PC's character, what their experience as players is, in comparison to the player for whom you're asking, etc.. If they're all playing, say, "Farmers" (there was such a profession in RoCo.II), well, even a mere Fighter would probably be unbalanced. If their characters are all ten levels higher than the player for who you're asking, then you may indeed want to give him a profession a bit more powerful than theirs. Same if they're all have ten years more experience in RPGs. Etc.
From my POV, nothing is really "broken" and it's unrealistic to ever consider that. Are guns broken? Well, indeed, compared to swords and arrows, they are. Yet they came into existence and use. Are nuclear weapons broken? Probably. Didn't prevent them from existing, though. In fact, any new creation can be broken compared to former ones: it's called progress.
So, are some spells and professions more powerful than others? Probably. Though, it also depends on the context: in the land of the blind, the one-eyed man is king after all. In the end, people adapt to everything: an invisible flying mage throwing fireballs may be powerful... the first few years. But after some time, not only would there be so many of them running (well, flying) around, having one in one's army wouldn't give such an edge, countermeasures would have been developed and anti-air devices (i.e. magic items) able to detect invisible would be widespread as well. After all, if profession X is so much better than the others, then every rich family will push one of their heirs to become one... And if SuperWarriorClass is way better than any other fighter class, then after some time, anyone able to become a SuperWarriorClass will be one, so being a SuperWarriorClass wouldn't give a party such an edge, as most other party would have a couple of SuperWarriorClass members as well!
Balance is unrealistic: the world isn't balanced, no world is. And true unbalance doesn't last: people copy, adapt and evolve. An "invisible, flying, fireball throwing healer that can control your mind" may be powerful compared to the rest of the world... until the character actually does something significant. Then, in a few years, except if for some reasons no one but the original guy is able to learn invisibility, fly, fireball-casting, healing and mental control (but then, healing would be unbalanced if no one if one person in the whole world can cast it), every single person with sufficient means would be able to do the same, and, since every rich and powerful person would invest into countermeasurese by fear of the above, magical items allowing to see invisible, fly, protect from fire and mental control would probably be available to anyone rich enough, making the original character powerful when compared to people with little means. But then, a guy able to get the best teachers and the best equipement is the same compared to people no able to get time to train, teachers or any kind of equipement.
As for balance
in a team, it depends very very little on the character's profession itself: players' experience, equipment, characters' social status, characters' levels, etc. would actually play a more important role.
Personally, I've been playing without the limitations of RM2's realms for decades, having merely Open and Closed lists, all put together, and I never got ever such "unbalance" problems, at least no because of that (I had others I had to solve, but it was more related to the points I aforementioned). In fact, even with such rules, I hardly had players willing to play pure spellcasters: only two out of a bit over one score of players. The reasons? Those who tried, abandonned then played mere semi-users just they noticed how painful and a headache it was to play one, since one had to manage hundreds of spells, know a whole new set of rules (I think it became better in RMSS but in RM2, the magic system was just a set of rules different and in addition to normal rules, that with EAR or BAR modifiers and all), parameters (how to read spell descriptions), manage more figures for a character (PPs using a different recovery system), etc.
My question to you, Cory, would be: so you think that going without RM's magic classification would because unbalance problem but did you actually
try doing so?