I am Polish but I have not lived in Poland for long time. There are some comments of people being scammed etc. which I have no reasons to doubt.

However, on the other hand, UK stag dos in Poland (especially in Krakow) are notorious for being a nuisance for the locals. I feel like as long as you and your mates behave like just a decent bunch lof lads without causing disturbances and issues and are careful of potential scams (which would be easier of course if you are not absolutely wasted and/or drugged) then you should have a great night. Which I hope you do as Kraków is a great city, in fact if you have time to actually walk around and visit it I would strongly recommend it.

Have fun!

EDIT: If you need help with anything Polish related while you are there hit me up.