But that's what mod reviews exist for. Of course users who buy them first (though I don't know why would you even pay for a mod that isn't reviewed by anyone yet and you're not sure the modder is capable of making working stuff) will risk losing money by getting non-working crap, but in case the mod is broken everyone else will know that. After all, people can always report workshop items to get them banned from the store.

It's the same as with Early Access / Greenlight games really, Steam never guaranteed that content you get from them would work.