My biggest hesitation on the guy now is that we just have been through a QB who lacked emotional maturity. Now Williams has faced far more adversity than Mac did in college, but the fact he broke down sobbing in the stands with his mother scares me a bit. And of course his size is a concern.
But that brings us to the difference between Williams and Manning/Luck/Lawrence. Physically, the later three were prototypical NFL QB. Williams, on the other hand, is very short. Doesn't mean he cannot be a generational talent, but it definitely makes him different from the other guys.