Yes it's very much quantity problem too. Europe is bunch of nation states. Which means that language and race/ethnicity are founding parameters of the most fun

Yes it's very much quantity problem too. Europe is bunch of nation states. Which means that language and race/ethnicity are founding parameters of the most fundamental and powerfull social construct on the continent. If you get 1% of immigrants, they are just guests. But 20% would be very problematic to the national identity, because they don't fit in to the matrix of ethnicity and language. But you can't treat them as non significant "other" anymore because they are everywhere all the time. So it's also a speed/integration problem. Most Brits don't mind "Normans" anymore. Many Estonians have beef with the very significant Russian minority that migrated after WWII. Of course current immigration is not anykind of actual problem. It's just that people are afraid of escalation and politicians seem incapable of actually doing anything.