They are perfectly safe to use at home and wont harm your baby. I think what the mil is probably trying to advise you of is that they can sometimes be tricky to use accurately and therefore can cause you undue worry. Like if you were using it every day and couldnt find the heart beat one day, you might panic, but it could just be that the baby has moved to a different position where its harder to locate its heart beat. Alternatively I think sometimes people sometimes hear the blood moving through the placenta and mistake this for the heartbeat, giving them false reassurance. Used properly and only occasionally, i cant really see how it can do any harm, providing you are aware of other signs that your baby is ok, eg, noticing its movements. Good luck!