1.) you cannot force the migrants to stay in the country they are allocated, without destroying the freedom of travel (Schengen‼). If there are no internal border-controls, they will just leave their aLLOCATED COUNTRY AND GO TO THAT THEY WANT TO LIVE IN, PROBABLY THE THAT ONE WHICH GIVES THEM MOST MONEY WITH MINIMUM WORK.
2.) you cannot send aliens into any country against the will of the population of that country.
The result will be ethnic with the majority forcing the unwanted people to leave as soon as possible. The only way to prevent this is an international oppressive police force, that is the loss of national sovereignity and individual freedom.
I have lived that through in Hungary in the 1950s, when the communist party took the half of out flat and put a secret police under officer in those rooms.
When there are no flats/houses immediately available to the aliens, the international police will have to make room available by requsitioning/confiscating private flats, houses, thereby violating a numerous paragraphs of the Human Rights declaration.