It's understandable. The plane can roll very quickly, so using the mouse for roll allows for finer adjustments compared to using keys or buttons. It's also intended that pitching and rolling is your primary means of turning, rather than yawing, which I like having together on the same input for that reason. Some games use yawing as your main means of turning, but I wanted to make a pitching and rolling game. I'll be exploring rebinding in the future but I suspect some configurations may not work well for this game.
The feedback is helpful, thank you!