Williams is, right now, the same player he was his freshman year of college. He has barely developed any. It didn't help that he had a reputation of being very immature, not the best learner. Also not helping was a horrible coach at A&M that played him in a two center system.
But he is dripping potential, though maybe not as much as some think.
Williams is not a better ball handler than Tatum or Rozier. Not sure how anyone can get that who watched him in college or anytime this year.
Williams is a good rebounder. He is neither a great rebounder nor an elite rebounder. And he will have a tougher time here in the pros than in college because he isn't physical or strong enough to move people. He really needs more lower body strength that I am not sure he will be able to develop. I think he was the potential to be as good a rebounder as Horford was at his age, if he ever earns playing time.
Unfortunately, I don't think we see his best until season 4 or so, if he is even on the team then.
As for comparisons to Harrell....completely different type players. I don't see any similarities.