Even in a server based game somethings have to be done by the client. The client always issues teleport commands to the server because pathing is a very difficult problem for the computer to figure out. Instead the client just sends a position update with timestamp, and the server treats your movement as teleport. Therefore it is easy to have teleport hacks since all you're changing is how much you've moved.

At any rate usually the most common 'hacks' are really exploits. That is, doing stuff that is legally acceptable in the game in an unintended way. It's very difficult to hack your client to say "I have no GCD", but perhaps you can find a tricky combination of abilities that somehow reset your GCD. It's hard to say "I have Godmode", but perhaps you can find a clever way to stack certain buffs/items to have 100% chance avoidance which is usually good enough. Exploits cannot be stopped architecture design, because what you're doing is allowed by the game.