Everything matters, but what matters the most is the competition, you cant compare a season at Roma without competition, playing as the lone striker, with a season at Juve having to compete with Tevez and Llorente.
So their Roma days are somewhat comparable, thats why it makes sense comparing them.
Again, i repeat that i never really rated any of them as finishers. They are not good enough to be our starters IMO, but Vucinic's case was easier back then, he took his chances and averaged close to 15goals for 4 seasons.
Osvaldo only had just one good season, just that!