What's missing from this discussion is the understanding that it's not a black and white issue. No modern game uses purely server-side or purely client-side state tracking. There are too many problems related to either. Hacking from the client side, and delays from the server side. The proper way to do it, which most games do, is to have both the client and the server running the calculations and just sending verification back and forth. That way everything is smooth unless there is enough lag that the client has to be contradicted. That can be minimized by having a short polling interval (.3 is incredibly long for a modern game), and by having a lot of well designed prediction. That's why in most MMOs and FPSs, everything is great and visible lag is nonexistent up to the point where the prediction and compensation can't handle it anymore, and everything falls apart.That's typically 300-400 ms ping, which nearly everyone in the world can manage to anywhere else given a good connection. Overall game design also plays a role, abilities, animations, and UI effects have to be designed so that there is a little leeway and instances where there could be visible desync are minimized. The bottom line is that SE just has shoddy netcode for this game.

It's not unfixable, but it does appear that it will take more work that just a few tweaks. Proper prediction and lag compensation is a very complex subject, and it's not something you just stumble on, you have to read the academic papers and understand what solutions the rest of the industry has come up with. The best bet in the short term is likely going to be to design encounters around the problems. Fast AoE dodging and precision positioning should be taken out of their toolbox. They should also up the polling rate to something more reasonable, which would have an immediate and noticeable effect.