Holland is a average league,there has been a list of goalscorers who have bagged goals for fun in that league and have come to the EPL and done nothing of note.
Kezman Alvez VanWinkle DeJong Depay at the top of my head who have flopped here in the last 10 years.
And the only success in that time frame has been Suarez Bony and maybe Kuyt who was prolific as a striker at feyenoord but eventually turned into a wide player at the RS.
Janssen's goal scoring rates for the national/under 21's team is also impressive. I hope we start making a move once Koeman is announced as clubs are starting to make moves early for players and we can't afford to be playing catchup considering our ambitions to overtake so many of these teams.