All of this discussion so far has been talk and no directly-stated concrete proof. I can safely say that Poland's UA is OP, and that I have evidence to prove it. In my 3rd (THIRD) game in BNW that I have ever played, I won a t292 DEITY SV as Poland. I have some experience in G+K emperor SVs, but the two games that I played on prince in BNW before resulted in losses, and that was using the Shoshone, so that gives an idea of how inexperienced I was at the time.
Now here's the unbelievable part: I didn't build ANY guilds, build ANY culture-generation wonders, or get ANY great works during the game - and yet because of solidarity I finished the game with maxed-out tradition, rationalism, and a level-3 tenet in order. My culture-per-turn at the end was a measly 51. To be able to neglect such a vital part of the game, and yet still win a comfortable deity SV playing as Poland while having little to no prior BNW experience is a clear indication of how OP the civ is.