I assume you are looking for a digital option, as this is a digital forum.
The Mamiya is robust and some of the lenses are top notch. However, any digital back on the RZ will suffer from a huge crop factor, i.e. only a small part of the viewfinder represents the image area, and worse still, the FOV of all lenses becomes much narrower. The widest lens available, 50mm, will become equivalent to something like 35mm on a 35mm SLR.
So the system is limited at the wide end, and larger and heavier than it needs to be, as well as suffering from other limitations of the RZ system. I used the RZ a bit with film and would still recommend it to a film user, but not for digital.