They can work but it takes a lot of practice. Nylon is probably the most difficult to make work in a hemmer foot or attachment. Sil nylon is a BEAST. I don't use them on sil at all.
The problem I have with hemmer feet (as opposed to an attachment) is starting it, especially with nylon. I usually start it as well as I can and once it gets going, I can control how much fabric I am feeding into the foot.
Here is a video of 1/2" roll hemmer on nylon taffeta. I was shooting the video with my phone with one hand and trying to sew with the other. What is missing in the video is how "engaged" your left hand ist as well keeping the fabric in the right place and feeding it along/keeping pace with what is being stitched.
http://www.youtube.com/watch?v=31EOwtBukGY
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