I was initially a big sceptic when we got Ten Hag from Utrecht for the same reasons you doubt him. Did not really know the guy, only experience with a mid table team like FC Utrecht, little charisma, and seemed unremarkable. Just another uninspiring appointment from our DoF Overmars who is friends with him, but Overmars put a lot of faith in him and gave him the resources (we broke the wage and transfer ceiling in the 2018/2019 season which started when we signed Tadic and Blind) and Ten Hag proved him right on the pitch for which I give him a lot of credit. He got good players from the board, but he also executed and turned it into a good team. He plays attacking and dominant football, made us hard to beat, plays youngsters, and I also think he's a good people's manager while being much more comfortable in his presentation nowadays. Most of all, the players enjoy themselves on the pitch and it translates into results, not more that I can ask. Nowadays we can give any CL team a hard time while we seriously compete to make it past the CL group phase each season which I'd never expected a few years ago.
In my eyes he is therefore totally not comparable to Frank de Boer. In the years we played under Frank de Boer, we mostly played very very boring football. We won 4 league titles with him, but the football itself, the level of quality, and even the intention on the pitch is night and day. Very workmanship-like football with no joy on the pitch. Decent and unspectacular. If you look at the stats we scored 93, 83, 69, 81, 79 goals under Frank de Boer in his full seasons at Ajax. Under Ten Hag we scored 119 and 102 goals and we currently already have 30 after 8 games which is almost unprecedented even in the history of our "farmer" league. With Ten Hag we are dominant in the way that we should be given our huge budget compared to the other teams in the Eredivisie which is not a given if you look at history.