I've found major differences in rebuilt parts -- depending on who the rebuilder was.
Some guys know what they're doing and use quality replacement gaskets, seals, etc
and others don't. For example, Arthur Gould is the best rebuilder of fuel pumps, water pumps
etc. and I've never had a problem with anything from him. Ditto with Klaus Wojak for radio
antennas.
Both of these guys have been doing it "forever" which also helps.
I've had rebuilt water pumps fail from local shops in less than 200 miles -- so I learned this
the hard way.
I've not personally had any bad experience with engine parts (camshfts, pistons, lifters, etc.)
but I've heard stories about all of the "junk" that's out there that's not up to original
specs. Stuff from China, India has an especially bad rep.
Mike