1 star doesn't mean "easy to master", 1 star literally means "easy to use in basic capacity". That describes Rein to a tee. Even a terrible player (like me!) given the patience and a small amount of practice, can function decently as Rein in the basic capacity. You shield your team as you approach contested positions, you use your projectile once in a while, you charge if you need to sweep enemies out of the way, etc. Basically WYSIWYG, you press a button and it does that function, and most of the functions are singular in purpose.