The UA is good, very good, but the UB is where Poland really shines. On a quick reading it sounds like the only thing the ducal stable gives you is +15xp towards mounted units (which is all but useless) and +1 gold per horse/sheep/cattle (which is nice but not ground breaking). Then you see that it's also maintenance free which, hey, is a nice little additional bonus. But then you read the small print and see that it is +15% production towards everything, rather than just +15% production towards mounted units. This is huugggge. Its like a free forge in every city, available earlier in the tech tree, which also gives you gold. It absolutely changes the game. Massive.