Creation quirks always sound completely busted until you actually try to write a plot around them. The best authors don't just slap on a generic mana cost and call it a day. They dig into the psychology. If you can make anything, what stops you from becoming a god? Usually, it's the user's own mental framework. I loved how in one webnovel, the mage could conjure perfect defenses, but his 'creation' was limited to things he could visualize down to the molecular level. A wall was easy; a living, breathing ally was impossible because he couldn't comprehend consciousness well enough to forge it. His power was a mirror of his loneliness.
Then there are the systemic limits. The magic might draw from the user's life force, or perhaps each creation leaves a permanent 'mark' on their soul, gradually altering them. I've seen stories where overuse literally turns the creator into an emotionless golem, a slave to their own power. The limitation isn't an obstacle to overcome, but the tragic price of the ability itself. It makes the power feel earned and dangerous, not just a convenient plot device.
Honestly, I get bored when the only limit is a cooldown timer. The quirks that stick with me are the ones where the biggest constraint is the character's own humanity, or lack thereof.