On the subject of this kid's limitations: When you're drafting a 19-year-old kid you're often looking at raw tools and character (that is, willingness to work hard to get better).
They've been following this kid for a long time, and Brad is especially familiar with him from his Indiana connection. I think they looked at him as a prospect that has one elite and rare skill (finishing at the rim) and figured the combination of physical tools and character were worth the shot.
In other words, I think this is similar to the thought process behind Jaylen Brown, another 19-year-old with a good character and work ethic combined with good physical tools, but (at the time) a lot of holes in his game.