It's pretty much all be covered already, but I will add to it...
The biggest issue is typically timing. When facing a pitcher with even reasonable speed, a batter must get their process started well before the ball is released in order to have the time to execute a good swing in time. Very often, people feeding pitching machines do not provide any pre-release timing mechanism for the hitter, which completely destroys the hitter's ability to execute a proper swing process and be on time.
The biggest issue is typically timing. When facing a pitcher with even reasonable speed, a batter must get their process started well before the ball is released in order to have the time to execute a good swing in time. Very often, people feeding pitching machines do not provide any pre-release timing mechanism for the hitter, which completely destroys the hitter's ability to execute a proper swing process and be on time.