There are multiple possible scales...it could also be 20-80.
Although, either way, 50 is typically going to be in the middle of the scale. Since 50 is in the middle I am not sure why you would think that means they "suck". Obviously, it means that stat or rating doesn't make them a superstar, but typically that means they should be a useful major league player.
Here is an Explained article on the 20-80 scale.