It's probably about the same between the two games. I know shiny female Salandit is one of the rarest, and there were a couple event shinies that you can't get anymore like Solgaleo. Additionally, things like Poipole and Type: Null aren't any rarer than other shinies, but they take an incredible amount of time to get, as you have to clear most of the story and you only get one chance per save file to soft reset for them.