Bullpen management might not be the most important piece of a manager’s job, but it’s arguably the most visible. Complaints around Bob Melvin’s bullpen management usually centered around going to the same ‘trusted’ relievers so much their arms burnt out and that he often didn’t have anyone ready as a reliever in the game was gascanning.
It’s also worth noting that good relievers, and good reliever performances, tend to make managers look like they manage bullpens well, while bad relievers and bad reliever performances tend to throw managers under the bus.
So judging a manager’s bullpen management acumen is not always easy, but what you can glean is their style, their patterns, their ideologies, with which you may be aligned or not.