Almost all questions of immigration boil down sooner or later to two questions: How many? Which ones?
This leads to an unpleasant fact. Namely, the larger number of immigrants a country takes in, the less immigrants can be treated as individuals and the more they have to be treated as representatives of groups. That means, for any kind of large scale immigration policy to be workable it has to set goals in terms of groups (national, ethnic, racial, relgiious) rather than individuals (or to make the criteria for individuals so stringent as to end large scale immigration).
The only way to make that workable is to keep track of how well or badly individual immigrants do in their new country (in terms of education, economics and crime) and then correlate that by variables such as age, sex, national origin and religion.
Most European countries are loathe to do this and this results in ghettos and no no areas and a degraded social contract.