I've played a few civs, here are my opinions of them. Some are slightly outdated, like India/Poland which got changes, but in the case of Poland I don't think that'll change much.
Netherlands - Goedendag is pretty good. Obsoletes it's greatest opponents (Longsword/Landsknecht), but not by a huge amount and it has interesting uses. Gets stronger after turning into a Tercio because it gets the anti-mounted promotion back without losing the unique one it already has.
As a wide warmongerer Netherlands I got Waag maxed with ease, so it was okay. Nothing special, but it does what it should do.
Poland - Pancerni are an okay UU, the UB is okay. Since it all benefits from Fealty (faith per kill, faith on ub, etc) and UB requiring not Barracks, but the same as another building that benefits from that tree greatly (Castle), it has some synergy. Poland quickly turns into a place that's hard to mess with, but as the yields aren't something special, there's probably going to be less reason to mess with Casimir. Neither unique is something really great, but they're not bad, just simply decent. It's just that both are worse than what most civs I played get.
Siam - Elephant Camp is not affected by strategic resource for CS ally policies which is disappointing but necessary (it'd be very OP) but it is okay even if RNG fails, I didn't use the UU because I ran away too hard after starting next to good CSs early game so I got bored and left
Portugal - very strong UB. Starting late medieval/early renaissance, you really ball. I balled so hard with my choices and Colossus + Petra, I decided to leave before I could use the ranged UU
Spain - Armada is good but it's full HP promotion seems like something you should abuse in a way AI won't be able to. I think this sort of promotion (2XP every turn in hostile territory) is bad for the AI and I'm not a fan of it as a result even if the unit itself does it's job. Still, it was fun and good in my hands.
Hacienda is very interesting and I like it. It comes late and shouldn't be placed everywhere as only some spots are truly worth it, but it's good. There's synergy with the base vanilla kit for both uniques, which is nice.
Austria - very good but also kind of late UB that I really liked having allied most of the CSs, the UU is only okay for how I typically play Austria, but for someone more calm it should be awesome.
India - two great uniques. I only played them before they got their yields fixed (too high because of bugs, nerf for Qila), but it's still probably going to be a Culture/Faith powerhouse. It definitely was overpowered before.
Aztecs - UU gives me free workers (even easier city conquest back in the day when it was bugged) and the UB gives more benefit from the UA. I like how it plays. UB really takes a lot of time before it starts shining, though. Notre Dame and Fealty Wonder helped.