Having used their grammar checking in Google Docs for the last while, I'm unimpressed. It does catch legitimate grammar errors, but the fact that it's just recognizing "normal" based on some corpus of data means that it also repeatedly flags simple completely grammatical things as ungrammatical because it's a mildly less common usage that's similar to a common one. The most obvious example of this was when I wrote "She shut the car door" and it suggested that I meant "She shut the car doors" instead. But it's not only that, it suggests "gotta" frequently in place of the phrase "got to". It suggests that I change things to contractions. It hates having extra descriptive words in verb phrases: it'll suggest changing wording like "The light went right out" to "The light went out".
It hates the phrase "had had" and always suggests just changing it to "had". It frequently suggests other verb tense changes for no obvious reason, usually suggesting changing a more complex tense like future perfect into a simpler one like future, but sometimes just changing tenses from past to present or vice versa for no obviously reason. And it sometimes even suggests ungrammatical things because it's mostly looking at sentence fragments rather than whole sentences, so sometimes when one of your clauses has to be worded with a certain tense because of its place in the larger sentence it'll suggest changing that to a simpler tense even when that simpler tense is ungrammatical. So a sentence like "By the end of the next week, I will have read five books" it might suggest changing to "By the end of next week, I read five books." (That's just an example, of course, and that particular one might work fine. but it's sentence like that that it frequently provides grammatically incorrect suggestions on.)
I've reported all of these sort of problems, of course, but nothing's changed and nothing's likely to as their application of machine learning to this problem precludes any sort of actual active debugging or fixing. The best they can do is change their corpus and try again, but it's likely to still have many of these same problems because it conflates commonness with correctness. So hooray! Crappy AI-based grammar checking everywhere!