In the majority of cases, it's actually just plain ol' racism / bigotry and not about immigrants per say.
They don't complain about immigrants that are white and from a judeo-christian culture, for example.
They also don't complain about Asian immigrants for example - or far less anyway. Antwerp, the stronghold of that extreme right party, has a section that is nicknamed "china town". Almost 100% Asian, primarily Chinese. It has Chinese stores, restaurants, etc. The signs outside are even in Chinese. Another section is nicknamed "small jerusalem". Same story. Jewish stores, schools, synagogues, etc. They only employ "their own" also. Just forget about getting a job there if you're not jewish.
But those sections are never part of their rants. No, they complain about "Borgerrocco", which is a nickname for "Borgerhout" where a lot of morrocons live (and that not nearly in the same density as the chinese or jewish quarters). Same with parts of the city where a lot of black people live.
So it's not really about immigration. It's about people of color and religions they disapprove off.
I'ld respect their opinions if they had actual arguments backed by statistics against immigration (of all forms, cultures, colors, faiths,...). Arguments involving economics etc. I cannot respect it when it is just racism.