Guys that skip college and head overseas for a year still are generally picked on their high school career and potential. Jennings and Mudiay were drafted in the lottery despite not showing much their 1st year. Here's Jennings year in Italy and he went 10th.
In the Italian Serie A 2008-09 season, Jennings averaged in 27 games, 5.5 points, 1.6 rebounds, 2.2 assists, and 1.5 steals in 17.0 minutes per game. He shot 35.1 percent from the field and 20.7 percent from 3 point range in Serie A play. In 16 Euroleague games, Jennings averaged 7.6 points, 1.6 rebounds, 1.6 assists, and 1.2 steals in 19.6 minutes per game. In the Euroleague he shot 38.7 percent from the field and 26.8 percent from 3 point range.
Here's Mudiay who played in vastly inferior league.
Mudiay managed just 10 regular season games for Guangdong, only returning to action for the team on March 1, 2015 in Game 3 of their semi-final series against the Beijing Ducks. He played in Game 4 as well, but Guangdong lost the best-of-five series 3–1. In 12 total games (10 regular season and two playoff), Mudiay averaged 18.0 points, 6.3 rebounds, 5.9 assists, and 1.6 steals per game.
So Ferguson sitting on bench most of the year for Adelaide and still being a 1st rounder wasn't a surprise. Exum just had one good junior tournament then shut it down and was high pick.
Even a lot of Euro guys, apart from someone like Doncic, didn't really play much senior euroleague, drafted because someone wanted the next Dirk, 7 foot white guy that looked like he could shoot like Bender.
I don't think Giddey has enough hype for him to sit and be a first rounder, but with watered down league and an even more watered down Adelaide he should get a chance to play and contribute. 10 points and 5 assists should get him there with an ok shooting percentage. If he sits he might get an early second round selection.
Ball will be the biggest bust since Bennett, would never have put those numbers up on any other NBL roster and depending how/if teams can do private workouts someone should get a bargain late lottery/middle first round. Everyone is always a good pick for Spurs for development and culture, and they have great history of success stories with big guards, he'd be ideal there. Knicks will show how they're worst franchise in NBA and take Ball with a top 4 pick.