McLellan's inability to adjust during the playoffs with the Kings is a valid complaint. When you are playing the same team seven times, your system either works reliably or it doesn't. And if it doesn't, you need to change the approach or adapt.
But when he has the chance to work with young players, he does a good job. I think he had a good part on Leon Draisaitl breaking out, considering his pedestrian D+1/D+2 as a 3rd overall pick. He was also there for a chunk of Joe Pavelski's young professional career. Logan Couture, too. If you want to look locally - Adrian Kempe started taking off and becoming an all-around player under McLellan.
He's not a player whisperer, but he does have a history of young players playing well over time with him, and he did take the Sharks to the conference finals multiple times.
But to me, this goes back to the internal belief with the Kings that, up until Laferriere, they have an inherent behavior of slow-boiling prospects. It happened under Stevens and Desjardins as well - they ride the vets. Young players can step in if there's an injury or a player plays himself off the team.
He's respected around the league, gets big contracts, and doesn't stay unemployed long for good reason. He's a good but flawed coach who failed to meet managerial expectations, and his time was up.
The Kings had issues before McLellan and they seem to be doing better/making adjustments after McLellan. But I think it's fair to say he wasn't the problem.