Horses for courses, belts do have a space saving advantage if tucked away right but it doesn't remove the bulk, only moves it.
Cost wise it's still comparable on the small Nema 23 sizes with Oldham couplings although if you design the Oldham couplings into the drive train you can also get smaller, less looking like Price Charles and also save money in that you only need a torque disk to buy.

This is a Taig mill I converted to CNC about 4/5 years ago using belt drive.



It was done this way because I didn't want to use the flimsy Taig mounts, it could have been more compact if I had reversed the motors but I wasn't worried about space, just wanted a robust mount. Cons of this setup are you are still reliant on the original bearings for thrust / preload unless you re-design them.

As an aside why when people do direct mount conversions do they hang them out on studs looking like a grandfather clock ? It's got to be the least rigid type of mounting known to man or beast.

Going direct mount if you put a bit of thought into it you can get a quite compact and rigid design.



This is the X axis off an X3 CNC conversion, Nema 23 motor but it's the longer triple stack motor, note the mounting which is only 60mm deep, this contains two angular contact preloaded bearings and the Oldham coupling.

Everything needed is in this mounting block, very rigid, completely sealed and far more compact that the pillar mounting that everyone seems to choose.

John S.