I only closely follow the NHL and NBA. Comparing the two, in the NHL you have most players drafted out of the CHL, it makes it easy to compare them. It's easy to compare players within each of the 3 leagues (OHL, WHL and QMJHL), and not too tough to compare between those leagues either. After that you can slot in the USHL, European, etc. players, but you've already got a good, reliable framework set up - once you've got your CHL rankings down, it's not too tough to slot in the other players. In the NBA most players are drafted out of the NCAA, which is a massive, sprawling league - there are 345 NCAA Div I basketball teams in 32 different conferences! The quality of competition varies massively from conference to conference, it's really tough to say how good players are, you're constantly having to compare players who've played against totally different levels of competition, so you have to rely very heavily on just looking at tools, on court production sort of gets thrown out the window. It's not like the CHL where you see all the top prospects play against each other all the time, it's WAY more spread out in the NCAA, making it much harder to compare prospects to one another and get that basic framework down. Further complicating matters is the fact that NHLers are more or less all drafted at the same age, while with NBAers they'll be drafted after anywhere from 1 to 4 years in college, so you have to try to adjust for experience in your projections as well, adding further error.
Also, in the NHL there are good leagues for the prospects to continue to develop in post-draft (they can stay in the CHL or whatever league they got drafted out of, progress to AHL or ECHL, etc.). In the NBA players are no longer eligible to return to school once drafted, and the D-league is a joke, for the most part guys have to immediately make an NBA team or they'll just be cast aside.
Finally, in the NBA you just have generally 1-2 stars per team, and 8-10 rotation players. In the NHL there are twice as many players getting significant minutes, and roughly twice as many star type players, it's easier to be seen as a success.