Small reflection on the question, hope it helps:
There are different mantras that can do different things on a physical, subtle, and spiritual level. In India, you have snake charmers who are reciting specific precise mantras to attract or scare away hordes of snakes (most of them deadly) that can usually be living in different buildings. We also have subtle purification mantras that like
om apavitrah pavitro vā
sarvāvasthāṁ gato api vā
yaḥ smaret puṇḍarīkākṣaṁ
sa bahya abhyantaraṁ śuciḥ
śrī viṣṇu śrī viṣṇu śrī viṣṇu
or invoking the presence of some personality like a spiritual master, his blessings, and so on. And then there is mantra like Maha-mantra which when practiced properly, invokes our true spiritual nature by which we are no more identifying ourselves with the physical and subtle body, but with eternal relationship and serve with the Supreme. Mantras like om Shanti and such are helping persons to calm the mind, but by themselves, they are not going to give us the real spiritual benefit we are all searching for.