I think Mayfield is good, but he also has a year of tape on him. The league will be carefully analyzing his tendencies and now, in his sophomore year he'll need to prove he can overcome himself.
As posted above, OBJ can be nothing but a help, as long as things are going well. But if he doesn't get his numbers, if he isn't getting the attempts he thinks he warrants, he could easily become a problem in a locker room that supposedly is trying to become more cohesive and professional. He's a risk just like Brown. If all goes welll, he'll be a huge asset. If not - Dayum! Get your popcorn and hide your kicker nets.
I personally thought the best thing Mayfield brought to the Browns was his "Tebow" factor. The ability to inspire the guys around him to ball out. Sure, he's good, but good isn't good enough in the NFL. Last year, when it mattered most the Browns fell short. I will wait to crown Mayfield and the Brownies until they show they can consistently compete, even win.