While the level at which this happens is arguable, that question is by and large a major point of this thread.
I for one say yes, it is unbalancing, but not in the way that you seem to be expecting. It's not unbalancing in a way that it gives one class a lot more power than any other class; it's unbalancing because it makes some classes LESS powerful (which isn't something I can prove by a single example of an overpowered build). By giving spellcasting abilities to all classes that are normally spell-free, it lowers the power of casting classes