When he was here the first time, Ian Clark wasn't very good. Lu didn't adapt and modernize his skating (to his credit, Clark was trying) until Rollie got here. Also I think his relationship with Luongo was a little bit too cozy. Lu's style needed to adapt over time much sooner. Then again there were too many cooks in the kitchen in that case but I digress.
What Clark has done in the meantime is help Bobrovsky rein in his freelancing style and raw talent. He certainly deserves some credit for that, but it's worth noting that Bob and McElhinney were his goalies for his entire stint in Columbus. McElhinney in my mind is what he is. Bobrovsky was so slide happy and aggressive that any technical coaching was going to a positive for him. Clark's biggest strength is supposed to be on the technical side. However, some of Bob's early technical changes were instituted by Jussi Parkkila, who is now the goalie coach for Colorado, during the lockout.
Another issue is that Bob melts down every year in the playoffs (last post-season was his best IMO but I thought he was still very inconsistent). And every backup for Bob has had terrible stats.
IMO this lends credence to the idea that the organization is letting him go because they have a bunch of newer European goalies coming up and they are usually very heavily coached in modern technique all the way from minor hockey. Despite modernizing his approach, Clark is very much an old-school goalie coach. It's rumoured that Forsberg and Koprisalo may have clashed with him in terms of style. And Forsberg has since been traded.
As for the Canucks, I am not sure how Clark will fare here.
For Marky, technical side is good for him as it is similar to Bob where he has a lot of raw talent. However, I don't think Luongo's mental game improved until Clark left. And evidently with Bob it is not a thing that Clark brings to the table. Unfortunately it might be Marky's greatest weakness.
For Nilly, this may be a great fit because Nilly just needs to stay in his game to be successful IMO, and that is probably what Clark is best at.
For Demko, he is less technical than he should be IMO and needs (maybe considerable) help with his skating. Could Clark pull a Bob with him? The issue is that Bob was a very powerful skater even back to Philly days, in fact too powerful. I'm not as confident that Clark can help Demko with movement patterns which is what I think he needs.