"Hooliganism" happens quite a bit here in the US actually between rival fans of NFL teams. I've seen it first hand many times at games here in the states. People even get shot in the parking lots, and stabbed in the bathrooms.
It just does not happen on the "mass level" as it does in Europe with organized groups of Hooligans.
As for atmosphere, as much as I love the NFL, I'll take the atmosphere of a European football stadium over it any day. The fans of European football celebrate and chear tgeir teams on better. Everyone is singing, large groups of people have drums and flags, the stadium is shaking from jumping up and down. You don't have that at NFL stadiums. At least not during the several dozen NFL games I have been to, or when I watch NFL every weekend on Sunday Ticket.
I miss going to Bayern München and VfB Stuttgart games, and Champions League games, as well as National Team games. Went and saw Italy play Germany back in 2001. Italy won 2:1, but damn was it a fun time, and great atmosphere.