Everett IMO has been a myriad of issues. He has yet to be truly healthy is the main thing. Without that you can't really settle in and get the feel of everyone around you. I can bring up Goff as a piece as well, if he was a part of the reason for Sean not being able to evolve the offense as Sean wanted, maybe Everett was one of those pieces. And really until the end of 2019 TE's just never seemed to really feature in Sean's plans, or they were penciled in but never showed up game time.
A big thing as well to me was he would make an all star play and follow it up with an awful drop. Not very consistent, or maybe being consistently inconsistent is a better term. I don't think you can expect plays drawn up for a guy who may or may not be there when needed.