Soo.. I was recently speccing a cheater who would kill people right after spawning by just shooting in the air/melee attacking the air. He'd always do damage to someone and eventually that someone would die.
There's really only two ways such a cheat could work; the server is either not authoritative about who is hit by your shots and/or the server is not authoritative about where attacks are coming from.
If it's the former like in games such as PUBG, that would explain the ridiculous peeker's advantage in this game when people are playing off-region with higher pings. There's recently been an influx of American players playing on EU region (they themselves ofc say that it's because of player numbers) who know how to abuse high ping in relation to peeker's advantage.
The basic idea behind that is that it takes your ping + server latency and frametime + your opponent's ping for your opponent to see where you are after you have movement. So, if you have a high ping and cross a corner, it would take decently long for your opponent to see you while you already see him and can shoot him. There's of course a downside to this; if your opponent crosses a corner, he'll see you before you see him. But the gist is that people who consistently play with a high ping learn to take advantage of this by avoiding close quarters and situations where they would cross a corner simultaneously with another player. Instead what they do is that they opt for mid to long range engagement and peeking from behind corner's (hence "peeker's advantage").
The reason that this is made worse by client-side hit detection (and also by aggressive anti-latency measures) is that it removes the latency from shooting your weapon. With full server-side hit detection and no anti-lag, it would take your ping's worth of time for the server to register your shot; which, in turn, means that a) the person with lower ping may be able to see you and react first before the server gets your shooting command or b) the person with lower ping can start strafing immediately upon seeing you, thus giving him a chance to dodge your aim.
If there's client-side hit detection or aggressive anti-lag measures, the shooting command will come immediately, thus nullifying the major disadvantage for the shooter from his high ping (having to lead his shots according to his ping) and making having a high ping advantageous in most scenarios.
So.. What's up with that?