My brother was a controller (now retired). He hired on when Reagan fired the controllers. I've spent some time in Denver Center and Seattle Center and know the system.
He describes it as a hard job to learn and an easy job to do. I think that is pretty accurate.
Air traffic safety is not about super-smart people never fucking up. It's about a process that catches problems and deals with them before they become a disaster. For example- the moment separation is lost, the alarms go off and a controller is assigned to the problem aircraft. He only deals with that one airplane, and his only job is to get that plane routed back into shape. So mid-air collisions almost never happen. When they do, it's almost always aircraft that are not under ATC control.
Quality is always about process, not about people. People make mistakes- that is unavoidable. Good process recognizes that reality and accounts for it.