Stuff like getting Rodney Harrison and cutting Lawyer Milloy took absolute balls of steel that most GMs wouldn't have.
Building the 2007 team as well was genius and isn't recognized enough. Poison pilling the Dolphins for Wes Welker, trading for Randy Moss when nobody wanted him, etc. There were some clunkers (Adalius Thomas, Albert Haynesworth, Chad Jackson), but that's going to happen when you're assembling 53 man roster over 20 years.
Dude could build a team. And I think people underestimate how much the last 5 years or so have been affected by changes to practice rules in the last CBA. No more two-a-days, off-season minicamps being limited, etc. The hallmark of the dynasty Pats was their preparedness, never taking penalties or turning it over, I think a lot of that was coached up in those practices that they just can't do anymore, so the playing field has leveled so to speak. I don't think he's senile, just that the circumstances Bill the coach is working with are vastly different than even from 2016. But despite it all, the defense is still really good and they're able to churn out all pro corner backs from undrafted junk practically every year.
I guess the TLDR of it all is that he was a good GM and great coach, though I think the evolution of the game insofar as restrictions on practices have crippled the competitive advantage that he had on the rest of the league so now glorified cheerleaders like Dan Campbell and Madden needs like Mike McDaniel are finding success with half the tools Bill had in his prime.