am181d
Adventurer
Sounds like they should be looking at 1e/2e style multiclassing to deliver the stated aims of balanced characters that are effective without making the GM regret allowing them. 3e stlye multiclassing is much more problematic.
In my experience, 1e/2e style multi-classing was RIDICULOUSLY unbalanced. The Fighter/Magic-User/Thief could basically do everything the Fighter, Magic-User, and Thief could do, and do it almost exactly as well. (Maybe they were one level behind.) The seeming counter-balance was that only nonhumans could multi-class, and they'd eventually encounter racial level limits.
(This also exposes one of the biggest logic bombs in 1e/2e. The long lived races advanced in all of their classes at once before hitting an artificial ceiling. The short lived humans could advance as long as they wanted in one class, then switch to another, etc. Clearly, that should have been swapped, except they wanted to model the fighter/mage elf of the basic game.)