I watch a lot of 49ers games and it's clear he's a product of Shanahan and the surrounding players. Most of the time he catches the ball, it's from play design with others creating space for him from pick plays and play action. It's rare to see him leave a DB in the dust deep or beat them for a 50/50 ball. He struggled badly without Samuel and when other players like Trent Williams and Kittle took games off. This was like when Antoinio Brown trolled Juju on Twitter when he left Pittsburgh saying he only got open because attention was all on Brown. Now we have people on the Juaun Jennings train thinking he's the next best thing when he's also a product of Shanahan. He's Bourne 2.0.
He is not a transcendent talent like a Rice, Moss, Julio Jones, Calvin Johnson, etc who can put the offense on his back. He will disappoint a lot of people as the primary focus in NE as they don't have other players who would be able to take attention away from him. Finally, McDaniels has not shown to be creative enough as an OC aside from when Tom Brady was his QB. Going from Shanahan to McDaniels would be a massive downgrade for Aiyuk.
Shanahan for offense is like BB with his defensive players - most of them are a product of the system and can be replaced.