The MMM is perfectly fine on 6s, I've abused mine on 6s and it handled it well. It's the really hard braking that kills them, I have my brakes down at 30% and I try to take it easy on the brakes. I killed my first one with really hard braking on 4s, my 2nd one has been rock solid for ~50 hours of use so far (about 1 hour of use on 6s and there's nothing left of my tires).
All I ever wanted was an honest weeks pay for an honest days work.
Maybe MGM forgot that the MOSFET can take a little over voltage just fine and I'm sure most companies that makes a 6S capable ESC uses 30V MOSFETs. What they need to do is work on a esier way to set up their ESCs .