Erm....I'll go against the grain and say that I'm not too sure about this one, to be honest. We could judge Batshuayi and Lacazette based on certain factors (Poch confirming his interest in the former, Bielsa recommending the former, etcetera; with the latter, it's his impressive goal record over multiple seasons), but this Janssen's coming off one good year in the Eredivisie, was contracted to a side in the Dutch lower leagues at this time last year, and doesn't look like he possesses an easily definable key skill (unlike, say, Vazquez, who if nothing else at least appears to be a nutmeg master, judging by his videos
).
Of course, the scouts and Mitchell are paid to watch him in games and probably pick up on things we can't as fans (off-the-ball movement, perhaps, or footballing intelligence), but from my lamentable Youtube-based standpoint he doesn't seem all that special; decent finisher, decent header, gets into good positions fairly frequently..but doesn't look exceptional in any of those categories, and doesn't particularly look stronger than most, or possessed of any innate physical attributes that recommend themselves immediately to the observer (As far as examples of this phenomenon go, Batshuayi is pretty powerfully built, and Lacazette is just really quick).
Soldado looked like a nightmarishly good finisher based on his Villarreal vids - shooting from acute angles, finding the corner with impunity, conducting audacious flying volleys and backheeled goals like they were nothing. Yet we saw how he turned out, confidence shot and physically outmuscled by most of the players he came up against, to the point where even his finishing disappeared. Janssen's coming from the Eredivisie, which makes the possibility of him doing a Soldado doubly scary because the Eredivisie, unlike La Liga, is just pretty crap in general (relative to the Premier League), and produces loads of Vlemincxes and Altidores (both Eredivisie top scorers in their time) for every Suarez or RvN that does emerge.
Unlike some signings that shall not be named (which happened in January 2012 and August 2014, to be precise), I think this lad isn't a panic low-budget move, and that, if we sign him, we'll have probably seen something genuinely of interest in his skills and profile (and he's not without an upside by any means; his finishing seems good, he appears genuinely two-footed if the articles are anything to go by, and he's a good lad by all accounts, willing to learn), an assertion which I'm basing on the fact that Poch went to see him play against Ireland, among other things.
But I can't hand on heart say that I'm not somewhat apprehensive despite that. I'll be happy if he signs and we fill our backup striker position early enough, but I'll hold off on asserting that he'll be an immediate success, if he does end up signing for us.