In fairy tales magic is reliable: if the leprechaun wants you to be a donkey, you're gonna be a donkey. No save.
Magic in that context is just the continuation of Nature by other means. And Nature is, if not exactly Good, then at least implacable and to be respectd.
In D&D, it's somewhat less reliable--the sufficiently tough, clever or spiritually advanced can escape it, sometimes you even have to target it and land a hit as if it were a mere superpower, like Superman's heat vision.
It's still way more reliable than combat (as "caster supremacy" conspiracy theorists are quick to point out).
In classic pulp stories and in games that try to emulate them like Dungeon Crawl Classics, magic is extremely unreliable. It is Things Best Left Untouched By Mortals. Results go all over the place, they can fizzle or backfire or result in discoveries.
As usual, the Rosetta Stone here is Tolkien vs classic pulp (Lovecraft, Lieber, Vance, etc):
Tolkien lays out a fairytalish paradigm of magic as primarily a moral force: creatures who use it are in tune with higher things. Magic does what it says on the tin because it is the tin: the way the world is supposed to be working itself out. Magic only goes wrong and becomes weird when in the hands of the unworthy (like The One Ring). (As the OSR Discord reminds me, Dr Strange, Dr Fate and The Force in Star Wars also all work on this paradigm.)
The pulp paradigm doesn't so much posit a difference in magic itself as a difference in who a protagonist of a story could be: the unworthy--for whom magic is a risky proposition--are generally the focus.
So you have Lovecraft characters trying to use forbidden lore to gain knowledge and power, Lieber has the morally somewhat Grey Mouser as a sorcerer's apprentice who can't always get a spell right, and Vance has the inimitably amoral Cugel as much abused by-, as using-, magic.
Tolkien might agree they were getting what they deserved from magic, he just might not agree they were fit subjects for a story. At least not one of his.
In that context, it's easy to see why the game with the famously unreliable magic is also the one where it says right there on the book that you're no hero.
Question: Does this mean cleric spells should be inherently more reliable than wizard ones? My instinct is yes.