All the surfaces in a clutch system wear: flywheel face, pressure plate face, friction disc, pressure plate pivots and spring mounts. Diaphragm and coil springs fatigue. In some systems, splines can wear as well. I like to: change the pilot bearing/bushing, reface the flywheel, very lightly lube the splines, repair/lube release mech, and fit new disc, pressure plate and release bearing.
Clutches can also be rebuilt though this is more common in HD truck applications. A rebuilder will check the splines, replace the friction material, test/replace torsion springs in the drive plate, and, resurface the pressure plate, rebuild the pressure plate's springs/diaphragm as required.
I have fit 100s of new clutches in my 35 years pulling wrenches and I have had one or two "bad" new clutches -- usually dragging due to warped spring mech or plate. I have also fit maybe 10 or so "rebuilt" clutches. They, unfortunately, had a much higher failure rate with two or three of them being duds.
YMMV