Duke vs. Boston College was a perfect encapsulation of why Flagg will be an underwhelming NBA player. He does 90% of his damage by simply powering through undersized NCAA 4's and 5's and getting free dunks because of awful team defenses that consistently fail to keep track of all of Duke's blue chippers.
The thing is, Flagg is just 6'8'' and not really a fast twitch guy. For a guard or forward, if your quickness is just OK in college ball, you will be slow in the NBA. Flagg very rarely gets past anyone, which would definitely concern me if I was planning to draft him. He just lowers his shoulder, spams pivots on his way to the basket and powers through these future plumbers.
He won't be doing any of that in the league. What is he in the NBA? I mean that's something you don't necessarily have to know if you take someone with the 15th pick, but #1 is a different animal, particularly when the guy is not a generational raw talent. Flagg is not a LeBron, where you think "We don't care if he's a rim-crashing point guard or a playmaking PF in the NBA, we'll figure it out later".
Can you insert him in the offense as a first option? Definitely not. He's not even the first option at Duke. Do you treat him as a do-it-all forward who fills the blanks with his length? Possibly, but is that really a #1 pick?