The issue is that with a magic system along the lines of pre-4e D&D magic or psionics, it's very hard to balance. Well, that and a 3e-style multiclassing system tends to encourage designers to think concepts can be covered by multiclassing and don't need their own class, which means a lot of neat ideas can't be played at 1st level.I prefer 3e's level-by-level system. It had some excesses, but it's entirely possible to curb the excesses and preserve the organic, even, natural growth of your character and their abilities. Things that have been mentioned about 5e -- like less front-loading and bounded accuracy -- help ameliorate the problems with 3e's system a lot, and have additional benefits to boot.
I don't know what the issue is with a multiclass system that is "like 3e, but balanced."