Kessel is a dominant offensive player. tremendous speed, killer shot. he generates offense like a demon.
His play at the other end of the rink is not great though. Often he seems totally disinterested in that aspect of the game.
On a properly constructed team, he wouldn't stand out negatively. But on a team who is questionable defensively his failings get magnified.
People expect the high paid star to be the saviour of the team, so he carries way more than his share of the blame on that.
The Kane comparisons aren't crazy (I am a huge fan of Kane, so this is a compliment to Kessel, not an insult to Kane.) A team like Chicago can have a guy like Kane because the strength of the team lets him play to his strengths. the team as a whole can support an offense first guy. They get better scoring than they would have if everyone was a two way player, by giving their offensive specialist free reign to do what got him drafted in the first place.
A team that could support him would be someone like LA. In fact I think if they could figure out a way to get him, they could dramatically increase their offensive output, and their defensive game is sooo strong those numbers wouldn't really change.
They are scary as it is year to year, but could you imagine them with an elite offensive talent in the mix too?