It limits him. It limits the offense. It puts a cap on how good the offense can be.
Does that mean he's worse than Wentz? No. At this point I expect Heinicke will come in and lose gloriously to the Packers 20-35 or something and everyone will say something like "sure we lost, but at least we looked better at QB!" and they'll be totally correct.
What it means is he is very likely not the QB of the future. I didn't want Wentz here and I think he's a disaster, but there's a reason he's here. The front office was trying to find a QB with an NFL arm. Unfortunately that's about all Wentz was good for, and this team isn't nearly good enough to cover for Wentz's shortcomings. We'll see if the team can cover for Heinicke's shortcomings any better, because despite his moxie he does have them, and they can be pretty glaring.