
I love those little things and own a IIIF I would never part with, but only because they feed my masochistic tendencies. Just let me make a little list of pain points:
Archaic loading process - Need to cut the film in the right length and curve - or you are running a very real risk jamming/damaging the shutter. Film loading from the button without any visual guides, loading a Leica M is for babies in comparison.
Separate rangefinder - You have to move your eyes between viewfinder and rangefinder to focus - you even need to focus the rangefinder with a lever under the rewind know! The rangefinder is typically dull if no recent CLA was performed.
Tunnel Viewfinder - a narrow tiny 50mm viewfinder without eye relief, parallax aids or frames. That is why many use an auxiliary finder even in place of the native 50mm. The viewfinder is typically hazy if no recent CLA has been performed.
Shutter dial - the dial rotates when the shutter is released, and you can't see the current selected shutter speed unless the film is wound. You can only change the shutter speed with the film wound, and you have to lift the dial to rotate it and make your selection - the shutter range is also archaic 1/25-1/50-1/75-1/100-1/200-1/500-1/1000.
Elmar Lens - Most come with the legendary and affordable Elmar 5cm f:3.5. While excellent if clean, you will have to change aperture from a ring around the front element which is a pain in itself an more so with a hood, and impossible with a filter mounted.
And there are other minor irritation points that will affect operation: Filmwinding by knob not lever, Film release/rewind lever does not automatically return after rewind, ASA/ISO reminder dial tops a 125, manual reset of the film counter, noisier than a Leica M because the shutter dial rotates rapidly when the shutter is released.
It is still as capable of making excellent photos as it was 80 years ago, just like a horse carriage can still get you from A to B and you can still write a novel on a typewriter.
Oh and they are inexpensive but cost as much as an M to have serviced if (likely) needed.
Cheers