Poland is only OP in one sense. There are quite a few civs with huge advantages. (Like 5-10 at least) There are at least 10 civs that have tiny advantages. So, it's definitely not an even playing field. But Poland isn't necessarily stronger than the other civs with huge advantages. (Like the Maya)
They are however virtually the only civ that can execute certain strategies. 7 extra social policies by the Information era allows you to complete an entire extra policy tree. This makes them one of the only civs that can take advantage of Jesuit Education without hamstringing themselves. (by delaying Rationalism, Tradition, Order or Commerce)
They may be the only civ that can pull off full Liberty + full Tradition by the start of the Renaissance without delaying Rationalism.
But this isn't OP, it's more an indictment of Jesuit Education, in that it takes 5 extra social policies before it's not an inferior strategy for science victory.
Poland also has one flaw in their UA. They get a policy on the turn that they reach the modern era, but you don't get ideology until the next turn. This prevents them from using it on Ideology. So it's not nearly as overpowered as it would be otherwise.
I think, really, Poland just outlines how broken some of the policy trees are... that you need 5+ extra policies before certain Ideology policies are worth taking. But yes, they're *very* strong. Definitely top 5 at all victory conditions. The ability to take Honor + Liberty makes them pretty bad-ass at warfare, the ability to take Rationalism + Aesthetics makes them amazing at cultural victory, the ability to take Rationalism + Patronage makes them amazing at diplomatic victory.
But what this really all says is:
1) Patronage and Aesthetics are under-powered if you have to prioritize Rationalism over them to be efficient.
2) Honor is under-powered if you have to prioritize Liberty over it to be efficient.
So Poland's biggest advantage is not having to choose, which is only necessary because the trees are broken. (IMHO)