I don't trust them because they haven't been around long enough to know what that weird ink is doing to the cotton for the long-term. Since the lines do sometimes come back after a washing (per my own testing as well as comments here) that tells me it's not washing away completely - maybe because it's basically been heat-set? They're not convenient enough for me to chose them over other marking tools that are tried and true.
I use my Frixion pens at work. It's fun to surprise people by erasing ink. I stick to tailor's chalk (when I will be ironing before washing it out) and blue water-erase pens (when I won't be ironing before washing) for my quilts & projects.