Amnesty rewards law-breaking, punishes law-abiding applicants and American workers and gives unfair advantage to residents of a contiguous country. That is all we need say.
Also unfair is to call them economic migrants, as if that is a negative. Much of the "better life" that U.S. immigrants have historical sought is based on higher living standards. For example, 19th century German immigration was largely motivated by wage disparity.
So we should be kind to Wet Back criminals. Ok, I guess.