There is a more nuanced argument against deportation as a policy. First of all it causes migrants to destroy their documentation and to be less coöperative with the immigration process. Second, some migrant countries refuse repatriation, which is currently an unsolved problem. Finally there's something to be said for immigrants to be registered regardless of status, rather than incentivising them to avoid authorities.

Now some of these problems could be solved, but there's a legitimate argument that the policy causes more problems than it solves.