As a test, I put 'What is wrong with you can not you do anything right' on an online notepad and had this extension check it to see how well it could spot errors. The results returned no errors. Obviously the grammar is horrific and there's no punctuation, but yet the extension claims no errors. Yeah, I wouldn't trust this on any important business. 1 star for being a decent spell checker, but there are a lot of those out there.
Edit: Dev, you're missing the point. Whether the error was on purpose or not, this app failed to correct it. And yes, someone with English as a second language and not familiar with contractions, could very well use 'can not' instead of can't or worse. The sentence written correctly would be 'What is wrong with you, can't you do anything right? or What is wrong with you? Can't you do anything right?' Yet no error was detected and no suggestions given for alternatives. If the quality of the app relies on the users inability to make unaccounted for mistakes, then it's not very useful. Or perhaps I should say accurate. I suggest using this experience to further polish your product to a better standard. When I check back after some time and I see that the product has improved, I'll update my review.