Honestly I don't think his blocking is that bad. He seems to have gotten much better at it
It's not that it's bad but it just isn't consistent. And you rarely see him actually control his guy. He's more of a "get in the way long enough for the RB to slip by" type of blocker. You'll get away with that a couple times a game but you can't build a running game around it and the TE blocks are so much more important to our running game than the average fan can possibly understand.
There's a misconception about our run blocking that we just maul the guys in front of us. There IS that element but these runs that look so simple are extremely well orchestrated, timed, and executed. If even one guy is out of sync, the work of 10 other guys that executed their assignment flawlessly is ruined and an opportunity spoiled. It's the difference between a two yard loss and a 25 yard gain. Tough to trust a guy that doesn't execute at a consistent level when the stakes are that high.
That's why the Patriots are so dominant with that two TE package. Gronk and Hernandez (now Bennett) were/are excellent in line blockers AND down the field threats. It allows them to create mismatches both down the field and in the trenches. VERY tough offense to defend because they can catch you in a defense that can be exploited by both threats and go into hurry up mode eliminating your ability to substitute. As much as I detest the Patriots, it's actually a thing of beauty to watch. So frustrating for opposing defenses and coordinators.