Any reason the GS-1 hasn't been mentioned?
At the 645 end of the spectrum, the Mamiya and Bronica are often considered comparable. I'm both biased (I own an ETR-Si, which I love), and utterly uneducated on the subject (I haven't so much as handled any of the cameras being discussed), so I'm the perfect internet commentor!
I'm geniunely curious if the GS-1 falls short compared to the Mamiya or Pentax 6x7s in some way.
I have a Bronica GS-1 system with several backs, viewfinders, lenses, and assorted accessories. It's a very capable system. The lenses are outstanding across the board. All of the shots in my gallery here were shot with the GS-1 on Ilford Delta 400 film. I'm certainly not being limited by the camera. Much the other way around.
For comparison purposes it would be considered an alternative to the Mamiya RZ67. They are both electronically controlled leaf shutter cameras with interchangeable lenses, viewfinders, and backs. (The RB67 is all mechanical.) It's a bit smaller and lighter than the RZ67 and is quite well hand-holdable, especially with the optional hand grip. On the other hand, the RZ67 has a rotatable back where the GS-1 does not. In order to shoot in vertical portrait mode, you have to rotate the entire camera 90 degrees by hand holding or flop the tripod head to the side. Even at a lighter weight than the RZ67, the GS-1 with a back, prism finder, and a moderate lens is probably a few pounds, so a substantial tripod and head is required to flip it on it's side. Bronica made a special rotating tripod mount, which I have, to rotate the GS-1 on top of the tripod but they are rare and usually quite expensive.
I think a GS-1 body with a 120 back, non-metered prism finder, and an 80mm lens could be put together for $500. But a single lens will soon seem limiting and well, we all know how one thing leads to another. Pretty soon you have quite a nice system, but you've invested some money to get there. If I were shooting 80% outdoor landscapes and 20% other stuff, I'd be all over one of the Fuji GSW670 or GSW690 models. It's hard to appreciate how comically big they are until you hold one in person, but they are much easier to carry around. My GS-1 spends most of its time firmly mounted to a camera stand rolling around in a studio.