The big issue is if he gets too comfortable running, and runs even when plays aren't there, or runs for 10 when there is a 30 yard pass there for the taking.
I'm not saying that will happen. And his running has been crazy good. It's just a result of seeing Browns QBs in the past getting "used" to scrambling from pressure to the point that they scramble, pressure or not.
Allen can develop as a passer, but isn't very polished yet.