I have never encountered a story with awesome plot and characters and terrifying grammar. Bad grammar goes hand in hand with simplistic plot, bland characters, the use of words smirk and yell every few sentences, and chronic inability to chose subject other than personal pronoun or the character name.
There are no drawbacks to good grammar.
Interestingly, I was once honour bound to read a story that had great grammar but characters and plot so, so atrocious that I couldn’t believe my eyes.
I am usually not against self-inserts, but it was mega, uber, epic quality Mary Sue that married the worst of the OP Mary Sue and the Pity Sue. Gods, I can’t convey how bad that story was.
So bad, that I remember finally finishing the last chapter in triumph with as polite comment as I could come up with, and the writer was like, “oh, wait till you check out the sequel!” And that’s when I fondly contemplated moving to man less wilderness without the access to the Internet.
Anyway, that story should have come with the broken syntax, lack of punctuation and confusion of there are and their etc, but it was grammatically sound.