I have no doubt that any fly of a reasonable size* will take a fish from time to time.

Some do so more frequently than others, however. There's absolutely no way to know which is which without having fished a fly over the course of years. I'm skeptical of anyone who tells me that the latest fly he just invented is the latest thing since sliced bread -- he may have just happened into ideal conditions for that fly the first time he tried it, and it may never take another fish again.

I've caught trout on a bare hook on a couple of occasions, but I wouldn't call a bare hook a great fly.

*By "reasonable size" I mean that you could probably fish an eight inch long muskie streamer in a stream with nothing but 6 inch brook trout every day forever and still not catch a fish; likewise a #32 midge probably will never interest a muskie. (There are probably a few other necessary features, like having a hook with a point, but not many.)