Just look at Sanheim.
He hasn't been ruined and he's still extremely good, but look at the difference in how he plays when he's "let loose" when down a goal vs normal, where he's more careful and reserved. Especially because he's not used on the PP and spent years on the 3rd pair being benched any time he made a mistake.
He's a Ferrari being used like a pickup truck thanks to incompetent coaches.
Or look at Ghost. He gets PP time but look at how often he's been screwed over in his career by being underused/demoted/scratched, when guys like Provorov are completely bulletproof while playing worse.
It massively limits their play and impact on games.
Imagine both of these guys if they were brought up by coaches who believed in them, trusted them, and gave them a little more carrot than stick.