Very true Piker20. I call my versions Semi Intruders as I only tie with Marabou at the front of the fly. This is because of the huge lose of flies on the rivers I fish in BC and cuts down on the expense of tying the more intricate true Intruder patterns. I find it not to make much difference as long as you can get the movement in the water. This can be improved by tying in a ball of Fritz or similar behind the head before tying in the Marabou.Ive done well on seatrout and bigger rainbows. A guy on YouTube uses them for ferox.
Many versions you now see online are not intruder flies as Ed Ward intended. His original design created a light fly, capable of casting on a #5 or #6 line that sank quickly but created the illusion of bulk and size in the water. Rhea was used because the fibres are so fine you can fit lots in without loosing cast ability or sink rate. When dry Eds flies have a distinct front and back (sometimes middle). The front fibres when dry stick out from the fly. The cheaper versions using marabou and ostrich and other fibres will not work quite as well as the original intention and are in my mind variations of a streamer instead of intruders.
I get a lot of designs for my BC flies from the site link shown and find a guy called Jay Nicholas very informative.
Caddis Fly Angling Shop
I have tied flies for the UK and just use the more traditional colours like Orange, Yellow and Black (Willie Gunn).