I believe there CAN be gum and enamel issues if you OP incorrectly. Remember that the remaining oil in your mouth after the pull can be full of toxins or whatever. If you didn't bother to rinse out well enough or brush afterwards on an ongoing basis, that could cause problems.
I notice that when I get distracted and go over my 20 minutes - sometimes 30-40 minutes, I get tooth sensitivity. Likewise when I don't rinse well enough afterwards. Just my opinion, but I do believe that holding the oil too long and not rinsing well enough on an ongoing basis just might lead to enamel problems or gum recision. Make sure you're doing it right!