Have any of you GMs out there used some if the nasties out of the Terrinoth sourcebook? I'm thinking of lifting a few to use as arena beats and the more 'chaos' or magic infused thingies as Sith Alchemy Abominations...

If so, how did the players react?

I've used them; I've even used beasties from WFRP3e. It's just handy to have another set of stats for things with maybe one or two unexpected abilities. It was nice to have something that no amount of player "system mastery" could overcome.

I will say they ran most successfully when I leaned into the weirdness / abilities and not into the statblock.

TBH I just need the weird stuff.... I reskinned Wampas waaay back for a jungle/forest planet to make a Swampa..I think there's a six legged reptile/riding thing that orc-like things use.... may just lift the beasty and the riders to make a primitive Gamorrean type culture.