It's kind of impossible for me to guess how useful these will be without imagining how they'd synergize with the rest of the unique aspects of the civilization and how those would encourage me to play, so sorry if this gets long, but:
Winged Hussar: I'm worried that Poland doesn't know exactly what it's trying to be and that the WH is another indicator of that fact. The UA calls for a mix of Science/Culture to really press its advantage, while the UB and UU call for a mid-game aggressive military strategy more in line with Domination. I guess it can be a sort of jack-of-all-trades civ, and that's fine, we probably have enough situational ones. Just keep in mind to choose whichever ideology emphasizes Science Culture and Domination, because Poland is still built for that to be sure.
As for the WH in particular, now that feels situational, but it is strong for it's time and looks awesome and has a fun ability - to a point. Sometimes you don't want to knock an enemy back, let's remember. But whatever, it should be fun to play with getting an opponent's back against the wall and squeezing them there. I look forward to the first time I see one of these force Carthage to retreat into the mountains.
Siege Tower: Assyria is definitely looking like the Hypercube civ for me personally. I don't normally warmonger, but they've got enough unique stuff going on that I'm going to have to learn, and all of that goes against my early-game instincts. To me the most important thing to remember about Assyria is that while they are very clearly Science/Domination, that science is a constant gamble and the civ has to be played in a very particular way (from what we can tell right now) in order to press it's advantage to the limit.
Because we're all looking at the siege tower and not at the Royal Library which gives literally twice the bonus that a Library does. Which is kind of huge, I'm thinking. Assyria will be rocking the science. But it also can easily mitigate the UA if one isn't careful. Assyria could easily and quickly outstrip most other civs in science and then not get anything special from taking their cities.
Which means that they are built specifically for beelining, of course! That might not seem to have much of an effect on my thoughts on the Siege Tower at first, but there are two things to think of there that I haven't seen mentioned yet. First: it gives you two great justifications not to bother with teching archery - the ST won't buff ranged units and you're almost certainly going to get it from a conquered city anyway so why bother? Secondly: it's kind of okay that the buffs don't go past medieval, because you're beelining yourself to gunpowder anyway. So all in all I think ST could very well be a game changer. It just might take a while to get used to in context.
Pracinhas: I don't really need to go into Brazil's synergy, as it's both obvious on the one hand and unknowable on the other (i.e. none of us have played with the new culture victory, so what can we really guess about it so far?) But I just finished up an Ethiopia game last night (cultural, tall, turtling) and can see the use of the Pracinhas. While I liked the Mehal Sefari (and more on them in a minute), war was still a distraction - and a nearly constant one as I was situated next to Attila.
Now, cultural victories have so-far been pretty turtle-y, to be honest, and I'm sure they've gotten much more interesting with BNW, but what I like about the Pracinhas is that you can goad your neighbors into attacking you, because it's only going to help you anyway. let them try to take you out and then push the battle back into their territory for extra Carnival points. Basically, I like them for taking an aspect of the game that had gotten stale and adding just enough of a twist to make it interesting. And for that amazing smoking snake story mentioned above.
Nau: Not much to say about it, really. Portugal is probably going to be my first game with BNW, I love trading and diplomacy. I love naval maps. This doesn't seem like the most interesting of units in that regard (that would be the Dromon, probably) but it has the possibility to be very effective. Still, Portugal's going to be all about the Feitoria from what I can tell right now.
Kris Swordsman: Indonesia looks like it could be wildly fun, though it is obviously extremely situational. As in, on a scale of 1 (France) to 10 (Polynesia) I'd put them at about 9.5. Still, I'm almost certain they'll have a coastal start-bias, and they are clearly built to rock an archipelago map (or small continents, or what have you.) So they're hard for me to guess about, because their promotions are random and we haven't seen them all yet, but their promotions also look incredible and point towards a pretty aggressive way of playing as Indonesia, which could be fun on the maps they are made for.
As for needing Iron to build them - so what? Indonesia's UA is designed specifically to reward you for REX. If you can't find multiple sources of Iron to settle under those circumstances then you're not playing Indonesia right, would be my prediction. (The Candi seems nearly useless to me right now, in theory, but I could be proven wrong.)
Berber Cavalry: You know how earlier I said that I'm a fan of the Mehal Sefari? Well these guys, while not super-flashy, look to be a much more versatile Mehal Sefari. Morocco looks like it's designed to be wide (and obviously financial) so I guess the synergy with these guys is that once you've won more territory you can hold it more easily (and with their range you don't need as many of them on alert.) These guys get my vote for "biggest pain in the hands of the AI" if only because they probably require the least amount of thinking to use them effectively.
Impi: And finally my vote for what will probably be the best one. Like Assyria and Indonesia, the Zulu greatly tempt me to stop worrying and love the war, but unlike the other two this one seems like the dream unit I've been waiting for. Why?
1. It makes the Pikeman something I'd actually build. Not just that, but build a lot of. While the food bonus from Civil Service is nice and all, CS has generally felt like a placeholder tech on my way to Education. Now it becomes central to my strategy.
2. Because it's medieval, it means I can still do a lot of the early-game builds I like to feel stable and get ahead of the curve. There's no good reason for Zulu not to go for the Great Library or what have you like with other warmongering civs, because that will just help you beeline anyway.
3. You can take a city by just spamming these. You don't need to worry about whether what you started building six turns ago is what you need once it gets to your front lines. They have the range for breaking city defenses and the melee for raiding/destroying enemy ranged units before they become a nuisance. This also means they can continue onto the next city quickly, happiness permitting. And the UA lets you maintain them for half-price!
4. Something I haven't seen mentioned yet. When the synergy between the UA giving a very nice promotion bonus and the Ikanda giving interesting new upgrades, you can actually spend your promotions on healing your units without sacrificing nearly as much. This, to me, is the underrated part of the Impi, and why I feel they could be extra-powerful in the right hands.
bonus in desert and friendly territory seems good.
