I've had a new gearbox (before my ownership), timk has, I think there's another one on here that has too. So that's 1% of UK sales already...
I believe the 722.6 5 speed box fitted to the C43 is more prone to failure because of the bespoke AMG software which was installed on the trasmission ecu.
Also these gearboxes get eaten up all the time because they have to handle so much power.
Obviously i would also think how the car is driven is a factor worth considering, being am AMG maybe the previous owner use to drive it quite hard a lot of the time, which will in turn lead to the gearbox suffering. Some people still have C43's with original gearboxes without issues.