Well, when they're taking duns, one of my secrets is to MOVE the dry. Cast beyond fish, skitter back to fish's lane, stop about a foot above the strike zone, and bang! That's a big secret to my dry fly success. And CD's don't skitter very well. At least my ties don't!
With emergers, it depends. I think rather than pickiness it's what stage of emergence they're focusing on. Swimming nymphs ride just under the surface before emergence, and a lot of times I think they focus on that. i.e. they're taking floating nymphs, not emergers per se. Floating a pheasant tail nymph works fantastic, unbelievably well actually, but it's real hard to keep them afloat. A tuft of CDC does the trick by keeping the body just below the film, and I've had unbelievable evenings doing just this. A comparadun, though, the body rides in the film itself. I get a lot of refusals, but it's close enough to get them to take a good long look and catch a few (more than if you had a pure dun pattern). But if they're actually taking true emergers, a CD works great.
As for spinners, comparaduns usually work ok, but not quite as well as actual spinners for me. And I can change a fly in about 20 seconds, so when the spinners hit I nearly always just tie a spinner pattern on.
Where a comparadun comes in, for me, is those situations where you have duns, emergers, and spinners all on the water at the same time and you have no clue what the fish want, or else every fish wants something different. In my experience, it works reasonably well for all of the above. And, yes, if you got different fish taking different things, you can catch more fish on an imperfect match of multiple stages rather than a more perfect match of only one of them.
Spring Creek, specifically during sulpher time, seems to be a stream where CD's work great for me on a regular basis.