When we hear the word unfaithful it brings to mind someone who is committing sexual sins against their partner with whom they have committed their life to. This is a bad type of unfaithfulness and it never ends well. You loose friends. You lose trust from many not only your partner. There are other types of unfaithfulness. You can be unfaithful to a friend who put their trust in you.
When we commit to someone or something we should always honor our commitment. If we can do it anymore then we should tell the other party and find a way to work things out. It can hurt people a lot to just change your mind.