Spent too much of his career pissing about in weaker leagues. And yes, I include Serie A in that. Similar to how Neymar is pissing about at PSG or how Verratti will never be rated how he might be somewhere else. Nobody gives a feck what you do in PSG, and unfortunately for him, he only really peaked in terms of his consistency when he left Barca and went to Milan and started killing it regularly and then to PSG. He showed how good he was at United at 34/35.. but he was 34/35. At Barca he had some terrific moments but he really didn't suit being in that team with Peps style and with Messi there, no matter how good he is himself. So while he was brilliant before going to Barca, he lacked consistency at the very top when it mattered because the Serie A was just easy for him, and then went to a team that didn't suit him at all, then went back to Serie A and then to Ligue 1.
As a player, the actual level he played at, he could've done a lot more IMO, and he was definitely among the very best strikers of his generation at his peak, a player that very few could rival. He just did it all in leagues where he didn't have to break much of a sweat to do those things, instead of possibly even reaching another level if he went to the premier league earlier (which is the league he was born for IMO).