There are many debates about that, and there are examples of QBs with low release points that are successful (although, that is definitely the exception, not the rule).
The issue is that he has changed his release point. And changed it from the traditional, over-the-top motion, to more of a sidearm motion. This hints at a potential injury as there are few, if any, QB coaches that would intentionally have a QB move from over the top to sidearm.
He may prove to be very successful with it, but the change of motion is one that hints at the potential presence of a shoulder or elbow injury. That, is something that could reduce velocity and/or accuracy, and is the concern that is being expressed.