He did a fine job developing young players. He helped develop Jake Guentzel, Brian Rust, Brian Dumoulin, Connor Sheary, and Teddy Blugers. So I’m not sure who you are referring to.
He’a never coached a high pick except Derrick Pouliot and it wasn’t poor development in Pouliot’s case.
He didn't develop Blueger, he destroyed Blueger. Blue was showing promise as a 3C and instead of grooming him more to take that role, Sullivan wanted more veterans and essentially gave anyone else a shot over Blueger and then started to give Blueger some of the harshest defensive assignments known to man.
Conor Sheary had moments of being good with Crosby but was mostly inconsistent and has bounced around since, not a good example there. Brian Dumoulin being developed by Mike Sullivan? LMAO
Some of these insane claims or credit you want to give Sullivan are comical. Jake Guentzel was on the 4th line at one point and Jake was so damn good that Sullivan had no choice but to promote him back up to the top line after first being sent down and then recalled. Bryan Rust these days went from a solid beast of a 2-way winger that can produce to being just an offense first winger that is a joke defensively and I say that as a big Rust fan.
Also, all of your examples are from 2015-16 or 2016-17 really. Since then, what? Blueger was basically destroyed to the point the Pens finally traded him and he bounced back with the Canucks but the damage was already done. It's a shame because Blueger really showed a ton of potential, but development, very poor development, can also severely stunt a player's growth and Sullivan has done that more often than your list of players he supposedly "Developed."
High pick prospect as a bar is pathetic. Just go with prospects alone, he's misused far more than he's "developed" into serviceable NHL'ers. In the last 7 seasons alone, his list is barely anything to sniff at - O'Connor finally had a breakthrough and then he shit all over Puustinen. Hallander literally said f*** this and left. He's barely wanted to use Sam Poulin and when he has, it's been strictly only for 3rd or 4th line usage at best. He did such a number on Kapanen, he's barely a recognizable player anymore.
Since 2017, the only player that has been drafted by the Penguins to even play for the team in any regular capacity is Valtteri Puustinen. Have the Pens drafted poorly? Maybe not the best, but the players that deserved a better shot in different roles were only ever used on the 4th line or the 3rd line at best - Poulin, Hallander, etc. Sullivan was more inclined to give a longer look to hacks, absolute hacks, like Harkins and White.