In 300+ hours playing Diablo 3 i never saw someone who appeared to be running a modded game. Of course people will always find in game exploits as they have with GTA5 on last gen consoles, i just hope they can keep people from hacking lobbies etc...
Like I said, the only "hacks" that can be done on Diablo 3 are done on your dummy client. It has no effect on the game, so nobody else would see them anyway. Stuff like Loot Alert and such don't impact anyone else.
In a game like Battlefield or CoD, even if you're on a dummy client, that client has information that could make you have an advantage. Since your client has the information of player locations (because it would be ridiculous to expect it to only give that information of things in your view rather than just displaying the ones you see and tracking everything), map hacks and spotting hacks are hard to detect. And, since those happen via DX Injection (intercepting the DX calls and modifying them), you can't really do anything to stop it.
I am going to suspect, since Rockstar isn't exactly an online MP juggernaut with a lot of experience dealing with cheaters, they won't have a set up where everything is handled server side and just displayed to the dummy clients. You're going to get people using trainers and such online. Those are rather easy to detect though, so if it is on Steam, they will get VAC banned pretty quickly.