As Rathwellin said.I disagree.
It's thematically good & doesn't seem unbalanced at all. I mean really. How often is a high level F/M getting interrupted anyway? Maybe one or two battles at most.
Note however, that this opcode (Creature Uninterruptible) was not tested by me before, and I haven't seen it in any other spell/ability. Chances are it doesn't work, or not the way one would imagine. I'll test it tonight.