Immigration in excess and esspecially in combination with exploititive or unenforced labour laws and mismanagement of other resources and infrastructure, can decrease wages, and cause shortage of key resources. For example, if there is no new housing being built, but there is very high immigration levels, housing prices will rise, and availability will be limited.

Well sure, but then why not build more houses?

Typically these quickly built housing is of such crappy quality that only immigrants will want to live there (because they can’t afford anything else anyway). This leads to the development of ghettos, with leads to the typical problems from crappy schools (that traps the kids in the lowest social class) to no cultural assimilation.