I think Koeman would be a safe bet in the sense that you likely won't get any surprises, positive or negative. He'd make the team perform well enough, probably above average, likely not more or less. However, I think Frank de Boer has it in him to be something special. Ofcourse someone that young who hasn't managed in England yet brings risks with him, but I'd be very surprised if he doesn't do well.
Well, he prefers to play a 4-3-3 possession style with high pressing. But he can be pragmatic and result driven too when needed. That is probably the complaint with Ajax supporters, as with their history they want to see exciting football always, but while Frank de Boer does value exciting football he tends to build in enough security into the team to get good results at all time, sometimes at the cost of exciting football. Then again, many Dutch teams sit back and defend against Ajax, so it's not easy to produce exciting football against teams like those in the first place. But he likes his team to be dynamic, defend as a team, attack as a team. Team above individuality. I think he's a more pragmatic version of Guardiola, as far as tactics and playing style goes. But it's no coincidence they're influenced by some of the same people.
GET HIM IN.