I have both Mamiya 6 with 3 lenses and a Hasselblad. The Mamiya is much easier to focus, especially in near-dark conditions. The issues of using polarizers or grads are non-issues to me--when mounted on an SLR the grad line is impossible to see anyway (even with the lens stopped down). I mark the dividing line on the outside of the filter and just estimate it. You can just bracket for precise position also.
The issue of rangefinder viewing and accuracy are very overblown--I have never been bothered by the final image. Perhaps you OCD types who insist on always printing the whole frame will care--in which case it's time for your meds. I have not found the Mamiya 6 to be fragile, and take it hiking regularly. I believe the reason for winder breakage is the fact that some rolls by some manufactures are spooled on very tightly and won't come off the reel at the film end. In that case, just don't force it. Anyway, I very much doubt the winders break with no reason. It is true, though, that there are no spare parts, and even bargain bodies now are going for near $1000. It is much riskier than a Hasselblad in that case. The Mamiya 50 is simply the best lens I've ever used. Period.