Yes, some migrants are more 'challenging' to integrate, especially to assimilate partly because some cultural differences mean that they do not want to assimilate and will not assimilate. This is key to what Europe has been experiencing.

Of course, for instance when Italians migrated to France that did create some tensions at the time but a generation later there was no difference apart from the Italian family names, because French and Italian cultures are very similar and, yes let's be honest, they both look pretty much the same.

Europe can and should pick and choose who to let in. As things stand there is no benefit for Europe in mass immigration from outside of Europe.