Looking at the record is way too simplistic. Sonics/ Thunder won 31 games before they blew it up, then they won 20, and then 23 games. The very next season they won 50 games. Now, I am NOT comparing the Suns to the Thunder, but it shows how deceiving a team's record can be.
Obviously, Booker and Warren are not KD, Westbrook, and Harden. I am fairly sure the Suns will not win 50 games next year, even with a #1 pick.
McD's picks have been reasonable. In 2013, Len was a reasonable pick. Noel and MCLemore are not any better, and if my memory is not failing me, those were the only other reasonable candidates at that slot. Sure, Giannis was available, and McD did pass on him. So did 13 other teams. Adams was there, McCollom was there, but a lot of teams passed on them too. in 2014, he nailed it with Warren. Bogdanovic was a solid pick too. He missed the mark on Ennis. Sure, there was Jokic, but everyone passed on him in the first round. He nailed it in 2015 with Booker. 2016, he made reasonable picks. Bender and Chriss were considered to be good prospects. Sure, I'd rather have Murrey, Sabonis, and/ or Heild than Bender and Chriss, but a lot of teams passed on those guys too. Jackson was a perfectly reasonable pick in 2017.
Bender, Chriss, and Jackson don't look great right now, but it's way too early to write them off. We've seen flashes from each one of those guys, so I am not ready to write them off. Jackson has confidence, Bender is already a decent 3 and D player, and Chriss actually had a very good rookie season.
I am not a fan of his trades, or the debacle that was Dragic/Bledsoe/ IT, or doubling down and taking Knight and wasting the Lakers pick (Imagine NOT having Knight, and having another mid-to late lottery pick this year!) I am not a fan of his coaching hires either. All that said, he is hardly a terrible GM, at least comparatively.