I just need to see more from Mayfield after the snap.
If his very clearly defined first read is open and there's little pressure, he's perfectly fine. But that applies to almost every QB in the league too.
But every variable added - if OL gives up pressure or if Baker believes there's going to be pressure, if the coverage on his initial read is good, if the defense shows a zone or man look when Baker's expecting the opposite - and Mayfield just doesn't handle it all that well.
The pieces around Baker - RBs, OL, TEs, WRs, play caller, scheme - are all so good that the Browns can beat teams if Mayfield simply doesn't give the ball away, but ultimately he needs to show more than that if the Browns are going to graduate from nice story to legitimate contender.
To put it another way, I firmly believe if you replaced Mayfield with a more complete QB, this would be the best offense in the NFL by a wide margin. Everything other position group plus the scheme and the guy calling the plays are all so legit.