Not sure what you’ve heard, but if you already own a Mamiya 6, you’ll find it is essentially the same camera with a rigid lens mount and a little wider frame.
Edit: I should add the only problems you’ll get with the Mamiya 6 or 7 are around the advance mechanism, with the 7 having an improved one (I think the 6MF is also more reliable than the 6, but don’t remember). The light curtain switch can also break. The grease in the lenses may shift, making focusing hard or moving to the aperture blades. All of these problems are rare and can be easily fixed today, as far as I know. Electrical failure is also rare but may be a more difficult problem to solve, especially if parts run out. All of this said, the Mamiya 7 is one of the most reliable cameras I have ever used, it gave me no problems in the 8 years I had it. Other cameras like the Makina 67 are far less reliable long-term (but don’t rely on electronics to work).