I have watched about two games worth of film on Poku, so I won't claim to be an expert or close to it. But on him, I tend to agree with Dejounte's analysis - I think players like him benefit much more than your average prospect from the lower level of compe ion around them. Most players that Poku faces are much shorter than him on average, much less strong than the players he'd face in the NBA, less quick. I saw him get a few blocks by chasing down his opponent from behind, something which might be easier in a lower league but is much less feasible in the NBA as a source for steals, IMO - only player I've seen that's really good at this these days is Thybulle, and he was always projected as a defensive savant. Again, I don't like to just list stats without context - where are those steals coming from, where's the blocks coming from? It's way different to be blocking someone from behind after he's blown you by, like I've seen Poku do, than to for example be fast or leaping enough to block a shooter's 3pt attempt, which so far I haven't seen Poku do at all. You might have film that sheds more light on this.
And rebounds is more of the same. He can use his combination of length and tall-ness to tower over the smaller players in the league, but once he gets to the NBA where the average player is close to as tall as he is, I'd bet money Poku's rebounding #s will go down. How is Poku going to fight a box-out with his lanky size and frame? How is he going to fight for offensive rebounds or crash the glass? Not to mention the very high injury risk that players with his frame have - even someone like Porzingis, by all means a player in the projection and trajectory Poku could be following, and someone who's evidently worked a ton on his body and bulked up, is still injury-prone and even now, isn't playing in his team's playoff series. Best ability is availability and all that jazz.
To be honest, if I'm drafting Poku, it's not for his defensive prowess at all. The things he's really special at, and the ones which I think could translate the most of his game into the NBA, are his vision, ball-handling, shooting. Now that is something where I'd be more inclined to your line of thinking "he's just too good at them for it to not translate" at some level. But defense... Nah. There are many clips (Dejounte posted some in the Poku thread I think) showing Poku getting manhandled and simply pushed aside on D by not even NBA players - and the worst part, which you haven't mentioned, is that he's likely to, and projected to, play the 5. With his frame. The five. Yeah... I'm good with Precious, tbh.
E: I forgot to mention something which I'd like to clear up. I don't outright believe Poku will be a bust, no matter how I've spoken of him. I think there's a real chance he can succeed if things go right for him and he develops... I just don't think the Spurs are the team for him, or rather, Poku isn't the player for the Spurs right now. I wish him the best, but hopefully we don't draft him and he can ball elsewhere.