False positives probably aren't a blocking issue. American citizens are already picked up in some sweeps, and even mistakenly deported on some occasions.

That said, they could just seed their system with a ton of fake users to make it more useless for ICE purposes.

That's only really an issue for the courts because, in practice, everyone is removable in one form or another. Maybe you are "illegal" or maybe the president just inspires some nut to remove you from mortality with a not-so-coy "if you don't like it here, get out"