I've never read a single EU book, so the defense that I'm attacking Disney and not the EU won't work. I can't tell you from personal experience whether the writing of those stories is good or bad.
Secondly, Mary Sue and Overpowered are not synonyms. One is a narrative term and the other is an adjective. A Mary Sue in layman's terms, is when the narrative can't handle the overwhelming characteristics of the protagonist or some other character. An overpowered protagonist isn't boring to watch when they can't overpower everything. The biggest example here is Anakin. He's force Jesus and yet was wrecked in AOTC a considerable amount of times. As powerful as he was, the narrative handled that power and still gave him a run for his money. I'm not saying that AOTC was a good movie but George handled Anakin's power decently at least.
This is why I find Rey boring. If you don't, then good for you, but she's most definitely a Mary Sue.
My comment about Mara Jade isn't one I have personally. I've heard good things about her character, and to be fair, EU Luke was also very overpowered. In order for someone to be worthy of him, she would have had to be powerful too.