I have a 34mph brushless in my e-revo(feigao 10L) and have a soft start on. I have killed all 4 drive shafts. So I know that a castle combo on 6s will destroy them rather quickly.
I have a MM/Neu combo in my ERevo. It's got a couple of hours of track/race time on it. It's geared for High 30's to 40mph. No broken parts in the drivetrain.