I think people are massively overestimating the difficulty of landing a knee. I am not much of a Captain Falcon player, so I'm not going to use myself as an example, but I would not be at all surprised if the better players will be able to use it about as effectively as they did in Melee.
Also, I'm almost certain the knee is actually more powerful in Brawl, which is why they made it a little more difficult to hit with.
The biggest obstacle facing Captain Falcon users, I think, is stale move negation. A mistimed knee is a problem, and it means you'll have to keep it stored until you're sure (enough) that you can land one.