Donald Trump has promised to deport criminal illegal aliens. That’s a popular idea, but it has a few practical problems.
Just because a Mexican citizen is in the United States does not mean they are here illegally. Your local police and prosecutors lack the information and background in immigration law to check whether a visa is valid and make those kinds of determinations.
The problematic part is Trump is proposing deportation upon arrest. Being arrested is not being a criminal. Arrest record and criminal history are two very different things. An arrest is not even to the level of an indictment, where a determination is made if there even was a crime committed. If you kick out the person who was arrested, that precludes even having a trial, since people accused of a crime have a constitutional right to a jury trial and to challenge their accusers. If this became an actual policy, it opens up the potential for corruption by law enforcement when just the threat of making an arrest results in automatic expulsion. Presumably that arrest means the person would permanently be barred from legal reentry.