Probably depends. I object mostly because I'm of the mind that, in most cases, nobody should be flat-out immune to anything (a few exceptions, such as mindlessness, make sense). If you're planning on letting casters keep their existing crazy defenses, or even some of them, then it's perfectly fine. Nowhere near unbalanced, in that case. Otherwise, it might be a little too good; it solves the problem of save-or-dies and save-or-sucks for the Barbarian only. Although if you're fixing those spells, too, then it goes back to probably being balanced since it won't be such a huge element of optimized gameplay.