Just saying "Dragon" could mean an overpowered character, but obviously they will be more powerful physically. There are ways to balance them out; for example, instead of just having unlimited fire, they'd have a firebladder that needs fatty foods in order to make more fire. You have a bit of fire, and once it's gone, you're left very hungry and in need of fatty foods. Another is that scales require precious refined metals, which forces a dragon to risk their lives to keep their armor clean and shiny. Scales are also armor, and are also heavy, so a scaly dragon is a slower dragon. A scaleless grey dragon would be much faster and could live far away from humans, but are much easier to kill.