I'm still not a fan of the failure chance as the means to that end, though. Primarily because it doesn't take the mage's skill into account. At higher levels, a mage should be able to stack effects higher than a low-level mage can, because he has more training, and he knows how to balance all those effects together without them interfering, or for whatever reason. Plus, I'm a little iffy of having class features that can accidentally kill you.
I tend to see a lot of magic acting on one creature as a bunch of people using the same radio frequency at once. If there's too much chatter, then any attempt you make to get a call out is going to be more difficult because of the noise. Not like a pile of volatile chemicals, and if you accidentally increase to critical mass, it goes boom.