CDC flies - highly air resistant
Flies tied with CDC (cul de canard feathers) have a tendency to get caught in the air mid cast and not lay out straight. This can be frustrating if you are fishing upstream and need the fly to land above the fish, but each cast falls short of the target.
EASY FIX - Fishing downstream can help and even if the fly and leader collapse, this can be a good thing with a downstream presentation as it becomes a slack cast allowing for a longer drift and a fly first presentation. Not my favorite, though but some situations call for this set up.
HARD FIX - or practice casting with a soft 5:5 rod and learn to lay out super light lines and light flies. Slow things way down and make sure the line lays out completely behind you before swinging forward. Each rod, line and fly has a sweet spot for speed, flick of the wrist, and power. Many of the best Japanese anglers make this look easy, but it's not.
Very light flies that don't spook trout.
Snooty trout may prefer a CDC fly.
Hard to see on the water.
Easily knocked off course by the slightest breeze.
Unable to support all but the lightest nymphs.
Get water logged easily, especially after catching a fish or two - requiring more frequent fly changes.