I have a late-80's complete rifle from Olympic. It works OK, though it's tough to disassemble (too tight) and the bolt face is so rough it makes the headstamps on the cases hard to read after 2 firings. I have 3 Colt uppers, 2 will fit on the Oly lower without issue, 1 will not go for love nor money. The Oly upper has never fit on any other lower I ever tried it with. The charging handle from the Oly won't fit in other brands of uppers.
Complaints about them seem to have dropped off in recent years, but all through the 80s, 90s and early 00's compatibility was a huge issue for Olympic.