I keep thinking of Rolando Blackman when I think of Jaylen Brown these days. Good player, made some All-Star games, part of some good (and some underachieving) teams but never became an All-NBA guy.
Played with a talented SF in Mark Aguirre too. Those 80s Mavs had 2 highly talented teams (two 50+ win seasons) in the 80s and were middling (38-47 win teams) the rest of their (Blackman, Aguirre, D.Harper) time together.
The guys I always think of with Jaylen are Jason Richardson and Caron Butler
As for Rob Williams, I see him as kind of a fringe starter also. I am not seeing what others seem to see. He needs to round out his game more. Right now, he is too limited. I am OK with him being the 5th best starter on the team or 4th or whatever.
I think Rob is really, really good at a limited set of things but those things happen to be very valuable things for a center.
He's an elite finisher. Very good at catching lobs. Overall very good roll threat.
He's got an insane block percentage, which is fun, but that does probably overstate his defensive value. He definitely seems to be a positive as an interior defender. Harder question about whether he's especially good as a team defender. I think he's improved in that regard.
He's fine as a rebounder, not amazing, but I think individual rebounding is overrated.
I think Rob could be better as a screen setter. But he does somewhat make up for that by being a very good high-low passer.
In summary I think as long as you don't need him to be a scoring option Rob does almost all the things you need from a center, and he does those things at a fairly high level.