AngusH, I agree about a lot of hyped guys being underwhelming. Coming from the NBA to the NBL is a big culture change, and a lot can't handle it emotionally.
Just thinking quickly about only the game side of things:
different refereeing rules and standards,
different style of game,
more of a team-based approach,
tougher and more physical defence, including targeting of stars,
having to play at both ends of the floor,
having to learn different defences and offenses,
more use of zones and hybrid defences,
being in the spotlight as a superstar so more pressure to perform,
different coaching style and standard,
weaker and less predictable teammates,
...
You’d have to be very tough - but flexible - mentally to overcome all that shit and eventually do well over here.