Duchene was slow to start with Ottawa for his first 20 games or so.
After that, he played some of the best hockey of his career and while he's obviously not on a McDavid/Matthews/Crosby etc tier, looked like a legit top center.
He played at something like a 90-100 point pace in Ottawa when you discount his first chunk of games where he wasn't producing at all. That's over the course of about 100 games and two seasons.
If I were a CBJ fan, and Panarin+Bob are leaving, I'm not sure I'd want to see them give up an unprotected pick 2020 pick, but if they do sign Duchene I wouldn't be worried about him not producing long term. What is happening now seems to mirror when what happened when he joined Ottawa. I'm not saying CBJ is going to be terrible if those players walk, just that unprotected picks are very volatile in the way the NHL is set up.
As far as Duchene being a cancer, there is nothing to suggest he is. While I have no way of knowing he's not a cancer, since I am not in the room, and there's been nothing really documenting his behavior behind closed doors in Ottawa, the burden of proof would be on those suggesting he is one.
At most there is the idea that he requested a trade from Colorado, but what is often ignored is that he didn't request a trade, more so he requested a resolution to a messy situation that had been ongoing for over a season. Sakic shopped him publicly for a long time. Second, the Uber video was widely characterized by ex-NHL analysts as being nothing out of the ordinary. Players bitch about coaches, coaches and managers bitch about players. Duchene and co just happened to get caught.