I think you can learn a lot about the players by watching the summer league games, but not by just looking at the stats. If a player is going to stick in the NBA in the near term, they need to be clearly better than the rest of the summer league. You should be able to see an ability to separate, get to where you want to get. If it is there, it will be apparent, even if shots are not going down. If you can't tell, if they look like all the other summer leaguers running up and down the court, it probably isn't there.
And even if a player does show some of "it", they still need to show they have the shooting touch required by the NBA, unless you are a 7-footer or something. You will need some level of touch to make it in the NBA, even if you can run circles around the rank and file summer league (example Springer, Walsh). Touch might not be there from game to game in the summer league, but it can develop (but not always).