It's really impossible to tell so far what some of these will look like, but:
Poland: At first I thought this UA was disgustingly overpowered, but then I remembered that the conditions for a Culture Victory are going to be different now, so it's really difficult to say. Still, a bunch of free policies can be huge, though probably not something the A.I. will be able to make the most out of. Winged Hussars sound aggravating to deal with and they can spam a lot of them with the UB, but realistically they also sound pretty situational, later-rising and avoidable.
Zulu: This could be horribly vicious, but could be underwhelming as well. I tend towards naval maps so it probably won't mean much to me anyway. Anyway, playing against Shaka in single player, I honestly picture the other A.I. players ganging up on him and asking for your involvement pretty early. He has to be aggressive to play properly at all, and aggression gets punished in this game. UU and UB look to synergize very well, so I can see why they're such a part of the discussion.
Portugal: I'm very excited for this, but like Poland it's going to be tough to judge until we understand the mechanics better. From the looks of it it more or less completely obsoletes the Dutch, who are currently one of my favorites to play as but whom the AI rarely understands how to use properly. Not too threatening, really. They're an economic civ without any real unique war-capabilities aside from the money they won't have until midway through the game anyway. Take 'em early and they should be a piece of cake.
Assyria: The only aspect about these guys that is at all letting me rest easy is knowing that they can't really plan out their tech tree with any foresight if they want to make use of their UA. COuld make them freewheeling fun to play as, but I picture them actually getting outstripped in tech pretty quickly when their stratagem meets resistance.
Brazil: I voted Brazil despite not knowing how their mechanics will play out simply because of the synergy in their UA, UU and UI, and the fact that they seem (like Portugal in some ways) very clearly designed to take advantage of the new systems. I can't say it any better than that when I read Brazil's description, I flashed back to every time I ran into Korea while trying to win a Science Victory. They are going to be trouble.