Here are his stats for the Illawarra Hawks Australian team. Obviously he dominated the stats but the efficiency is nothing to write home about. Looks like he's an even worse perimeter shooter than Lonzo 

I don't know how good the Illawarra team was...I'm sure he'll face tougher D in the NBA than he will there though, so the efficiency is a concern. Maybe @gouki88 can tell us more.
LaMelo was super unimpressive. The standard in the NBL isn't very high. Like, at all.
To give an example of that, let's look at the two last NBL MVPs: Andrew Bogut and Bryce Cotton.
Andrew Bogut went from a 4PPG 5RPG washed up big-man in the NBA to averaging 12/12/3/3 in just under 30MPG in the NBL. He might have won it for sentimental reasons in part, but you can see the questionable quality of the league when a toasted old big guy goes on to dominate here.
Bryce Cotton has won the 2 MVPs either side of Bogut's win. He won putting up 22/4/3 on championship teams. Not too bad, plays sort of like a dollar shop Allen Iverson. His play in the NBA? 23 total games.
Even if you take an NBL success story like Joe Ingles, it isn't promising. He took the NBL by storm, doing pretty much what he does in the NBA - averaged 15/5/4. But he took around 5 years to transition from the NBL to the NBA.
So finally, we get to Ball. An EFG% in the
30s (!) and weak defence (1 total block as a 6'7 rangy guard is really poor against the level of competition he was playing). That just screams be careful, at least to me. I don't want him anywhere near the Celtics. Has the exact same issues his brother did