The point highlighting his record without Tom Brady has been made numerous times.
And that leads us to a place where perhaps we don't really know what his pure Head Coaching acumen is. He slowly improved the Browns to a playoff team in 4 seasons. They cratered in his 5th season, but the question remains as to how much of that was driven by the cloud of relocation to Baltimore over the organization.
Then he started 5-13 with the Patriots. Brady took over after Week 2, and we all know what happened over the next 2 decades. And we all know about the cheating.
So, one might argue that there's more robust evidence that he's a lesser pure Head Coach than contemporaries like Bill Cowher, Andy Reid, Tony Dungy, or Jon Gruden. Yes, there are more rings on Belichick's hand, but these guys had sustained success across multiple Q.B.'s in their tenure.
My worthless opinion is that he is an exceptional defensive strategist and a solid H.C. He's somewhat around the level of Ditka as a Head Coach....he'll come in, the veterans and guys who don't buy all the way in have to be rooted out while he gets his kind of locker room. They'll slowly build up to the kind of team he wants to have, and the team will have big runs when the right Q.B. and O.C. are in place. Most organizations (and college programs) don't have the patience for that kind of slow build.
That 100% of the N.F.L. teams with openings passed on him last year is telling.