Hi, welcome to the community!
My weakness is being able to stay committed to a story long enough to finish it. I come up with a concept that is vaguely interesting and I will go, "OoOoO fAnTaSy?! SiGn Me Up!!!" It doesn't help that comics often take years to finish and I can't work on two comics right now, arghh.
Other than that - I'm not good at writing characters that are bitchy for the sake being bitchy (characters like Regina George basically) or that are otherwise bigoted. I don't know why I find it hard, but they often kind of become caricatures of shitty people than actual shitty people.
Art wise - I'm not good at drawing certain angles of people - particularly when the perspective is focused from above and when it's focused below. I kind of have a grasp on backgrounds, but I'm not that good at it either.