I'm simply looking at managers who recently (or little less recently) moved away from the Eredivisie and tried their luck elsewhere. And the success rate isn't great. Is it an indicator for Slot? Not really, but one way you're gonna look at it is to compare with other managers from the same league.
It does depend a lot on the nature of the tactics.
One of the interesting postmortems of Ten Hag at Manchester United was that his tactics are too open - playing a sole pivot, pushing his fullbacks up to overload, a brand of possession football relying on transitions and pressing, with a sweeper keeper keeping a high line at the back. It worked in the Eredivisie because the quality of opposition he would face were not physical, not athletic, not mobile enough to take advantage of the gaps that such a tactic leaves behind.
Ten Hag has brought that same tactical setup to Manchester United and the BPL and this has resulted in Manchester United conceding, on a per-game basis, the highest or close to joint highest number of shots on goal in the league. Teams in this league are fitter, more tactically savvy, and are far quicker to exploit the gaps left behind by fullbacks pushing up with a sole pivot as a creative destroyer type 6. It's far too open, and teams have figured it out and are exploiting it accordingly. This is Ten Hag being stubborn but Manchester United are one of if not the most porous team in the BPL as things stand.
The good thing for the Slotter is that he will have seen first-hand how such tactics translate, and he has a recent cautionary tale to guard against this.