Gerald wrote (and I comment between his lines)
o Except for the Mamiya C series lack of interchangeable focal length lenses.
There exist the Mutars for Rollei cameras. I have both of them and they work fine with my Flex 3,5F.
o Clunky in rapidly changing light conditions having to transfer light meter info to camera.
The light meter of the Flexes is easy to use and gives a valid exposure /stop combination in a second.
o Except for the Mamiya C series and some Rolleis the lens resolution is not all that great. Avoid those cameras with 3 element taking lenses.
The Planars of the Flexes ar really good lenses. The come close to 100 lp/mm in the center and around 60-70 in the corners.
o Film loading is slower than 35 mm.
That is true if you use an automatic camera like a Canon 33. If you use an older 35 mm camera, the Flex loads not much slower.
o Parallax problems.
Rollei cameras have a genial parralax correction. Even with acessory lenses the problem is solved.
o Square format.
That is not a con, that is a pro ;-)
o More prone to film transport problems.
I used more than 1000 films in my Rolleiflex and had only once the problem that the camera did not find the start of the film.
o Harder to focus particularly in low light.
The bright screens of the more modern Flexes / Cords (and Seagulls too) make focussing easy, even in low light.
o Leaf shutters need servicing more often.
I bought a ~40 year old Flex which works still fine. It got never a CLA. Do you really think that more modern camera still live after that period of time? At least the light sealing materials dissolves.
o Overall somewhat less robust in design than 35 mm cameras. Too many points where dust can enter.
I did not use my Flex in a sand storm, but I dont't see any problems with dust.