The problem is not necessarily immigration, per se. It’s who is allowed to immigrate, and in what numbers. Are they willing to live peaceably with their neighbors?
Are they assimilating, by which I mean, do they maintain their own traditions, but adjust them to the general prevailing norms of the liberal democratic society into which they have chosen to move?
If they’re not doing this, what kind of crackpot liberal democratic society accepts large numbers of immigrants from cultures radically opposed to the ideals it cherishes?