Honestly I think Miracles in this game are pretty great. From a support perspective at least, I made a holy priest to support my boyfriend through his first ever playthrough of all three games and Dark Souls 3 was one of the smoothest. I think the only downfall of Miracles in DS3 is that you don't really get any good ones until the Catacombs of Carthus, and you ride on Lightning Stake & Great Heal for some time. Though once you get both of the sunlight spells, and stuff like caressing tears & lightning storm, it's fantastic for zoning enemies and keeping any summons you have in the fight.
It's definitely not as high damage as Sorcery, or as staggering as Pyromancies, but considering the poise bonus you get when using Talismans it really can pack a punch and provide some unavoidable damage to people who get too aggressive in fights.