Comment by MindSpunk

How does it know what isn't visible? Can it handle glass? Frosted glass? Smoke? What if I can't see the player but I can see their shadow? What if I can't see them because they're behind me but I can hear their footsteps? What if I have 50ms ping and the player is invisible after turning a corner because the server hasn't realized I can see them yet?

To answer all those questions you either have to render the entire game on the server for every player (not possible) or make the checks conservative enough that cheaters still get a significant advantage.