Prolonged pacifier use can cause a babies teeth that are growing to be be pushed outwards or inwards or move around out of locations from where they're supposed to be.
It's perfectly okay for young babies of a year old or younger to use a pacifier to soothe them and even help prevent SIDS in younger babies.
But if the pacifier is continued to be used when the babies teeth really start growing and you continue to let the baby use the pacifier from age 2 to 3 and onward then it can mess up the baby or toddlers teeth.
Prolonged pacifier use can also impair speech and cause a toddler to have speech problems and take longer to speak.