None of these players are great but that's beside the point, the point is that this level and profile of player will score goals, it's not a particularly hard league to score goals in and it's not a shit league either. In my opinion the reality of the top leagues is that overall the levels are close, it's a matter of fine margins and mainly suitability. If a player fits with his own team and with the tactical culture of the league then he will most likely be successful.
If you take Immobile as an example, he struggled at Dortmund after having a pretty good season in Italy, he wasn't a different player and the Bundesliga isn't a vastly better league(if it's even better), it's just that he was a bad fit.