The cost of immigration, especially from outside of the EU, is catastrophically high, and remains that way, even for 2nd or 3rd generation immigrants. A paper studied the cost of immigration in the Netherlands to be around 400 billion euros between 1995 and 2019 (I can only find the paper in Dutch from the Renaissance institute, performed by several PhD / processors). The people arriving here from outside of the EU are often unskilled, in certain cases illiterate, and come from a cultural and religious background which proves to be nearly impossible to integrate, let alone assimilate.
As a result, in Brussels and many cities all around western Europe, it's riddled with ghettos, people living outside of the system, in poverty, with ideas that aren't compatible with our society (about topics such as religion, rule of law, sexuality, equality and so on). In my own neighborhood there are parts where as a woman you can't walk the street without a headscarf or you'll be harassed. And I'm not even suggesting what happens if you're openly gay in said streets.
Even multiple generations down the line the situation doesn't improve.
Why, in countries that are supposedly democratic, should mass immigration be the one thing that is shoved down our throats, without having any influence or say in it?
Side note: Belgium has a very flexible and generous social security system. We are known for providing unemployment benefits without any limit in duration, and with too little stimulus and followup, resulting in a lot of people simply exploiting the system. This is why immigration and the access to said system is a sensitive topic. We are also famous for having bungled the topic of integration for decades, resulting in people having lived here for ages and still not even speak the local language.