At Bayern II, he had them scoring the most goals they have ever scored in the league.
At Utrecht, he was also DOF as well as manager, so he actually set the foundations for them to be able to succeed after him.
Also, his Ajax team equaled a goal scoring record (fewest number of games to reach 82 goals if I recall), that was set by the Cryffs great Ajax side in the 80s, a feat even Louis van Gaals Ajax didn't manage.
His Ajax team also scored the 2nd most goals in Eredivisie history scoring just 3 less than the record Ajax set in the 66/67 season.
Other than Frank De Boer, what other successful Ajax manager has had a terrible history outside of Ajax?
Counting backwards from ETH
Keizer was sacked from Ajax and now manages in the UAE.
Bosz was sacked at Dortmund and then Leverkusen and now lies 7th with Lyon.
de Boer
Jol
Van Basten went 27-18-27 at Heereveen, then stressed out in months at AZ
Adrie Koster bounced around with stops at Brugge and an assistant job at Stuttgart, and is currently managing Willem II where he's soundly average.
ten Cate was so wanted, Chelsea bought him as a coach, was sacked along with Grant, resigned from Panathinaikos when they stopped paying him, then wandered around the middle east.
Danny Blind
Koeman
Co Adriaanse had middling success in brief stops at AZ and Porto, anonymously floated through Salzburg for a stop, got sacked at Twente, then resigned from an advisory role at Twente
Jan Wouters never got a full managerial gig except a couple years at Utrecht, where he had one good finish followed by a series of mid-table finishes before walking out
Morten Olsen managed Denmark for 15 years and failed to qualify for more major competitions than he qualified for
Louis van Gaal
Not going to go any further back, because I'm tired of the subject. Jol and de Boer are the only 2 managers there since LvG that managed at a high level. Jol was on his way down when he was there, and de Boer flamed out (after many on TFC begged for him).
Is ten Hag different? Will he be majorly successful at a bigger club than Ajax? Its a resounding
maybe from me. Again, I'd have taken him cautiously. But I'm not gonna sit here and act like he's the savior we missed out on. The odds are he isn't.