I have never seen a NTC3 diff fail.
I do not own the car, so I'm only speaking from what I've seen & heard. Plenty of people are blowing diffs, busting drive cups, and busting drive shafts with the high power engines. Just about everyone locally is raising hell about those 3 problems. I've also seen a few posts on various boards about those same 3 issues. It is my understanding the diff gears/rings & drive cups have to be replaced with steel units. I don't know what they're doing about the driveshafts.
I was actually considering buying a NTC3 as a backup car until I heard about these problems. While waiting for Associated to correct the problems, Mugen announced the MTX-3. That was a no brainer for me.