I think its all relative according to your skill level. I would agree that for someone with very poor skills, the fly wouldn't make much difference. That is to say that the angler with poor skills wont be able to put the fly where it needs to be and make the fly behave the way it should. If you put the same two flies in the hands of an experienced angler, I think you will see a marked difference in the catch rates of the two.
With all that being said, I dont think there are any flies (except Jeans fly in the previous post :tongue that wouldn't catch a fish if presented well. Some flies will just catch more than other flies in certain instances (depending on movements, floating or sinking characteristic or colour etc), and it is the job of the angler to know which fly to use as which time.
Around the steel no tortur'd worm shall twine, No blood of living insect stain my line;
Let me, less cruel, cast feather'd hook, With pliant rod athwart the pebbled brook,
Silent along the mazy margin stray, And with fur-wrought fly delude the prey
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