I watched the customary Janssen YouTube last night that showed all of his goals last season (both for AZ and the Netherlands national team). Must've been about a dozen plus goals with his left foot, a dozen plus goals with his right foot and 6 or 7 headers. He seems to score all types of goals, poachers finishes from rebounds, first time shots, goals where he shifts it onto his right/left foot and then strikes it early and goals where he gets across/in front of the defender to get on the end of a cross. He also looks remarkably brave. A couple of his goals saw him going in to get the last touch when he knew he would be absolutely clattered, but he had that desire to score without worrying about injuring himself, there is also a header or two of similar ilk where he put his head into where the boots were flying.
From the goals it is clear that he has two good feet, the ability to find space and also work space for himself, he can shoot with little backlift, is confident in his shooting ability, has good body strength and also while not being fast he does seem to have that little bit of explosiveness that allows a forward to get that shooting opportunity. He looks similar to Kane in that he is physically equipped to play with his back to goal, he is shorter than Kane but looks a bit stockier and naturally stronger. It looks as though he likes to peel out wide to come back inside to get a shot away just as Kane does - however whereas Kane seems to prefer peeling out to the left, it looks (from his goals compilation) that Janssen prefers to peel out to the right (I guess explained by the fact that Kane is right footed and Janssen left - so both of them peel out wide to then cut back in on their stronger foot). Just like Kane, Janssen also seems to hit a lot of shots across the keeper, low into the corners. He is also a confident penalty taker and varied his pens to either side of the keeper, down the middle and also sometimes low sometimes high.
Where Janssen is slightly different to Kane is that he seems to want to be right up front, he also gambles to get across/in front of the defender when a teammate gets into a good position out wide, whereas I think Kane tends to hang back when this situation occurs more akin to a typical number 10. IMO as well as being about as good a Kane understudy as we could get, Janssen also looks like he would compliment Kane quite well if we decided to go two up front (either in a 4-4-2 type formation or with Kane withdrawn into the number 10 role in our current system).
From the goals video I see no reason why the player would not be successful in English football, in fact it looks like he is tailor made for English football.