Sheppard is a shitty leader in the same way that Beckett is a shitty doctor: Because the writers are asshats who don't do their research and ignore little things like, say, continuity and character development. That I find Sheppard's flaws less egregious than Beckett's is purely a matter of personal taste.

That's the characters, mind you, not the actors. I think both actors are quite talented, and should not be blamed for the crap the writers inflict on them.