Pre-EtH, Ajax last won the league in 2013/14. They went four years without winning the league before EtH won it 3 times in 4 seasons. How they doing now?
Pre-EtH, Ajax last won the KNVB Cup in 2009/10. They went almost a decade without winning it before EtH won it 2 times in 4 seasons.
Pre-EtH, Ajax last made the CL knockouts in 2005/06. They went on to a semi-final (minute from final) and round of 16 finishes under him.
This isn't nothing, and if the above were true for everyone, especially given your mention of the weakness of the league and Ajax's relative spending power, then account for why it took EtH to produce their first CL knockout appearances in 13 years. Account for why the other coaches before and after weren't sweeping up the cup since 2009. Account for why there's a sizeable four year period without even winning the title.
It's easy enough to say this or that, especially with seeing what we're seeing now with his United, but don't let that discredit his actual work at Ajax, which should not be minimised.