If scoring 28 goals in 70 games on a god awful Buffalo Sabres team, or 17 goals in 48 games on a weak Winnipeg Jets team is "down to earth", I'm plenty fine with adding that player.
For me, Kane's biggest issue is injuries. Attitude problems or not, his personality clearly fits very well with this team, and I don't think he will create any issues. I just worry about him getting hurt. He has already been injured as a Shark; he took a few games off, came back, and he hasn't quite looked like the same player since. He's still great though.
He's on a playoff team that made the SCF 2 years ago and they just swept a team in the 1st round with him as the top line LW. In my opinion, that says a lot more than whatever happened in the mess of an organization that was Buffalo, or Winnipeg, which probably wasn't much better. His attitude problems are done. Guys like Pavelski, Burns, and Thornton love him. I couldn't care less what the 2014 Jets or the 2017 Sabres say about him.