Zeiss Ikonta 521 6 x 4.5 folders aren't very trendy, maybe because they seem too far removed from what most people think of when they think of film cameras these days. Lacking in amenities compared to more modern cameras, they're equipped with folding optical finders (no focus aids), film advance and shutter-cocking are separate operations, and there's not so much as a rapid-wind lever in sight. But that also makes for a very compact camera, and compared to operating a view camera, they're models of convenience and simplicity.
Shhhh! Don’t speak of those wonderful secret Goldilocks cameras!
I buy them whenever I can.
They are rigidly manual. And you have to perform five different actions for every shot, and do it right. Often without the aid of a meter. They need light to view the red window too, but not too much.
That might be what keeps most buyers away. Preferring the bigger negative of a 6x6 or 6x9 folder, “if you have to go through all that trouble per frame”. But that misses how folders really work and what they are good for.
You are never going to get as good alignment as with an TLR or SLR. So DoF is of importance.
Fiddling with an RF is anathema to fast moment to moment shooting.
For slow shots a laser RF or an external RF is far more useful.
I have three that I use as “backs” that I load with different kinds of film or ratings, and put them in separate compartments in the bag. Still takes up less space than an MF SLR with a normal lens.
You can get several for the price of a single tattered example with build in RF and still more if you compare to larger format MF RF cameras.