Oncey,
I talking about my experiences in dealing with fans, in large numbers, from countries all over the world.
Of course there's issues throughout the world with hard core ultras in every country.
The fact remains that the English, at times, are pre-disposed to helping create these scenarios.
As someone mentioned - if you pitch up somewhere in large numbers, pull your flags out, get hammered & start chanting about the IRA, ISIS and whatever else you're looking for trouble - and these aren't considered the English Ultras who are actively looking for trouble.
Most other countries can send large groups of fans to foreign countries, where they pitch up in huge numbers with flags, get drunk and sing songs that don't incite other groups to violence.