One of our PCs is a Swashbuckler. I should eventually buy XGtE but in the meantime we've been using the UA version.
Another PC is a Centaur. Originally it just used the statistics for a human character. We ret-conned it later against the UA version but not wanting to really recalculate anything or give the PC a net advance, we only added the Hooves weapon (ribbon) and the Equine Build property (encumbrance bonus, climb penalty).
I am not generally looking at UA as stable/usable material, but if something comes up that the players wants to play that is not explicitly available in the PHB, and if I recall some UA article covered it, then I can bring it up. My players usually aren't aware of the existence of UA, so they don't come up with "can I use <this> from UA?" requests. The Swashbuckler was my idea when the Rogue reached 3rd level because the player was a bit skeptic of criminally-sounding archetypes (Thief and Assassin) and had no interest in magical abilities (Arcane Trickster). Centaur was a player's request for purely narrative reasons.