Well at least police in England has done such a perfect job during London riots, aye? If there's anything to be said about hosting Euro in Poland/Ukraine, its that security forces are quite capable of handling anything that happens, because they're wont be pussyfooting if **** hits the fan.too risky with no safety procedures and useless police/stewards to stop the violence when it breaks out.
I seriously doubt you would have any trouble with the locals whatsoever just by going to Euro. Unless you don't know how to behave yourself. Then again, as proven during WC in RSA, very cheap beer and equally cheap, pretty hookers can get to people's head and some may start acting like they're kings of the world.