It would be interesting if we had more people contributing data with high mileage on specific motors.
Yes, there is a newer 'M600 variant' coming out, like there was an updated M500 brought out 6months ago or so. I'm not sure on the specific differences.
There are a pretty good # of M600 based bikes out there with seemingly good overall reliability, but of course - with some exceptions like the one-way clutch going bad (or getting overgreased by users).
If you're buying a pre-built bike, I wouldn't worry much about it - the motors are relatively readily available and much cheaper vs name brands, which the 'newer' M600 variant will use the same mounting points as the existing one, so in the worst case - you can swap motors if/when eventually needed, and there is at least some parts availability out there for internal parts.
I am curious if anyone has a source of general mileage vs failure data as it would be very interesting to review.