Notably the risks aren't simply that this will identify the undocumented its that it will provide a pretext to disappear almost anyone who isn't white.

The question is does any given non-white person look enough like one of over 10M people to get falsely flagged given agents a pretext to remove almost anyone they stop. Facial recognition of one person against a large enough database will almost always provide at least on possible match this is especially true if it matches against possible aliens and not against possibly aliens and citizens. EG a query of both will likely turn up Betty Sue Smith is herself and maybe a possible match with Known Deportable but a search against only aliens may return only Known Deportable with agents dismissing any claim by Betty that her ID must be fake.