The Crimson Binome
Hero
The stakes are the same: If you mess up, the game might be less fun.The stakes are just different. Worst-case, messing with fluff will make your campaign come off as a little silly or internally inconsistent. Screwing up mechanics can render the game unplayable.
You're probably not a professional game designer. You're probably not a professional fantasy writer, either. You're probably going to make a mistake at some point. The difference is, it's easier to fix a broken mechanic than a broken piece of fluff. If you don't like how a mechanic is working, you can just change it, no questions asked. If you don't like how a piece of fluff is working, then you need to ret-con it, which is way harder to do while maintaining continuity.