I've said it before and I'll say it again:
While in Philadelphia he took the Eagles to the NFC Championship game four years in a row, one year to the Superbowl. When the roster aged he kept the team in playoff relevancy for another few years.
When he went to the Chiefs they rattled off a 10-0 start and turned into perennial division winners.
Off the top of my head I can list John Harbaugh, Pederson, Sirianni and McDermott as active NFL head coaches from his coaching tree.
That doesn't happen by accident. Yes, he has his flaws. Yes, Mahomes put him over the top. How many times have we seen talent squandered? Reid is at the least a good coach.