There is some evidence that prolonged use of a front carrying that allows the legs to hang straight down a la Bjorn or Snuggli puts a lot of pressure on the spine and hips of the infant. That's one of the reasons some people prefer a MT or Ergo-type carrier; they are wider and so bring the baby up into a seated position, taking pressure off the hips. Whether this is definitive or not, I have no idea. I really don't know what amount of time is thought to be too long. Perhaps shorter trips in the Snuggli are fine.