Yeah Avatar had a technically overpowered MC, but he didn't have access to all his abilities right away - he had to work for them. And suffer quite a bit along the way for them. He had many chances to die due to his weakness at the time, or give up because it was too hard, etc. He only made it to the end because of the support of his friends and the people around him, and only ended up practically all-powerful because of some deus-ex-machina spirit turtle shit (and Nanoha is the same deal more or less). That is not the case with the kind of OP MCs we're talking about right now, who pretty much need little help if any at all and don't have to try very hard to overcome situations because they're OP from the get-go.
I have a problem with OP MCs too, it's just way too easy for them. It goes from "hero's journey" to "who cares, he'll kill anything that gets in his way so easily, so why bother watching".