De Boer has done a good job in my eyes. He hasn't made Ajax into a dominant side this year yet, but Dutch clubs always lose their best players. This year he's trying to cope with the loss of Blind in midfield. I think he's done great continually using youth every year, Ajax their team is very unexperienced yet always challanges for the title. This year Ajax is a bit of shambles, but I'm not willing to write this down to de Boer only. I think he's a great coach, winning the title 4 times in a row isn't just luck. They've made some bad transfers and this cost them after PSV got their sh*t together.
It's not compareable to Cocu. In his first season (last year) he arguably had a better selection than Ajax (at least equal). The season turned out a disaster, ending 3rd after a winning streak. It was PSV's first year they really implemented their youth, Cocu was still learning and made mistakes. The side was very young too after alot of changes in the summer. They lost van Bommel, Strootman, Marcelo, Mertens, Matavz (off the top of my head).
In his 2nd year he kept his most important players: Memphis, Wijnaldum, Willems and bought de Jong and Guardado. They've been great and Maher is finally stepping up too after a bad first year. They have a better selection than Ajax this year but Cocu has learned too. He doesn't try to play 'Dutch school football', he's got a rapid front 5 with target man de Jong. PSV are great on the counter but aren't the best in posession, this isn't what you normally see from Dutch teams. I wasn't impressed with Cocu last year but he's developping, it's too early to say where he will end up.