I always feed on coco coir yup they often eat some it is not a problem. I have had a snake become constipated due to substrate ingestion (cyprus at the time) but she is a very odd case (one eye and mostly blind) The only substrate that I know of that has ever caused issues with ingestion is paper towels. To my mind it isn't worth the risk as it is the same as paper for the snake there is no clear advantage to make up for the minimal risk. Some have their reasons for using it and those reasons out weigh the chance but I don't see it myself.