He sucks because he's simply not a good character who basically should've died a long time ago.

His personality is that of a cardboard box, you can replace him with a dry sponge, and the story wouldn't suffer.

He doesn't do anything that is remotely impressive despite the screentime he gets, and he's just there as a lover for Erza. Nothing more.

Having Erza talk over him during the GMG fiasco made him look really pathetic.