Harrison doesn't have the size or athletic ability to play in the NBA. He had very a very tough time against guys that could match him physically, as that was his only way to create space. He'll be guarded by guys 3 inches taller with much more length that are more athletic.
He's one of my favorite Johnnies ever....it's just not in the cards for him despite all of his skill. Dom probably stands a better chance as Sampson proved to us this year. He doesn't have Harrison's skill, bu there is nothing holding him back physically.
Sir'Dom will make it imo. He not only has the physical ability but also can handle the ball at that level. He needs to make his jumper more reliable, but he's versatile, relentless and can be a guy that takes the ball up the court at times. It gives him a bit of an edge with his versatility. I've learned not to write Dom off.
As far as Harrison goes, I think he certainly should try basketbsll for a while at the pro level. Make some money whether it's here or Europe. I do feel though, with his fire and love for the game, his best bet is looking into coaching down the line, at the college level. He has matured a ton, has court smarts, and knows the game quite well. I see him possibly ascending as a coach down the line.