Not only were they generally unflattering (unless you were Denise Crosby) and tended to ride up in people's areas, they stank to Warp Factor Nine. According to costume designer Bob Blackman in this archived interview with the BBC unearthed by Fast Company Design:
"Spandex retains odor, so there is a certain part where if you’re wearing them for a long period of time, you can’t really clean all the smell out, and it becomes a little bit annoying. And it also retains the odor of the dry cleaning fluid. It is, on a day-to-day basis, unpleasant."
The tight spandex fabric also gave the cast members recurrent back problems, which necessitated replacing them with looser wool uniforms starting with the third season.
So if you've ever watched the first couple of seasons and noticed that the crew seems a little stiff and awkward, keep in mind it wasn't just the lousy scripts. Not only was everyone physically uncomfortable, the entire bridge probably smelled of Woolite and BO.