Or at least they should not be able to manual block super fast out of a couch.
I think it is a balance issue.
Not to mention incredibly unrealistic.
You mean going from horizontal to vertical in a split second? Not sure.
For balance issues I have more of a concern with those who couch, and block with a shield at the same time and the arrows you shoot at the horse get attracted to the shield like a magnet (or the shield magically covers the entire horse).