Most "Stars" in college get exposed defensively in the NBA in my opinion. There is no NBA offense in College to judge their defensive abilities, and that's what makes it difficult. You get to see some things when you bring them in for workouts before the draft. That's when they get a good feel.
I was at the University of Portland game vs. Gonzaga (2 blocks from my house) where Olshey said he decided right there and then he was going to draft Zach Collins because of mostly his rim protection, but also because of Collins' natural feel for the game. Olshey knew then that Collins was years away physically from contributing positively in the NBA, but that when his body developed, he would have a skillset that other big men in the NBA simply don't have. There were only about a 1000 of us in the gym and I didn't see Olshey. I didn't see what Olshey saw, but I was there to watch Terry Porter coach. I was saddened at how bad he was and left at halftime.
Guess my point is that Olshey was going by his instincts and visual and didn't need any game film. I feel the same way when I look at baseball players and a lot of basketball players. It's why I keep supporting Marcus Smart. Also, seeing someone in person is the key. It's just not the same on film, tv, etc., though I only needed to see 60 seconds of Batum when I knew he would be excellent. He immediately reminded me of Jamal (Keith) Wilkes. I felt the same way about Harkless, Kersey....perhaps it's only that style of player I recognize....instinctive on the defensive end, ability to glide on the Offensive end with a long reach, and always sees the right play to make. Unfortunately for Batum, he's too soft and timid. Could be a French thing, the yoyo-ing that Nate did to him, or both. But, he let's his outside emotions effect his job.