They aren't reliable at anything I guess, but for English I have nothing else, and they are better than nothing. I do wish they would use a more effective way of highlighting their suggested changes, such as italics for new text and strikeout for deleted text.
Unless you are paid by the word, I struggling to think of why you would use an AI to create new text. The facts will be wrong, the tone won't be yours. "If I had more time, this would be shorter" is a truism here - AI can spit out an enormous amount of text in a very short time, text could be cut down to a fraction of the size with a bit of effort.
As I said that doesn't matter if you are being paid by the word. If the goal is to be paid, who cares where the words come from if the reader laps it up. But if you enjoy writing for it's own sake, and if you are doing because you've found the discipline involved writing something down in a way others will understand is an excellent way to sharpen you're own understanding, then the less a AI is involved the better. Sadly I need a proof reader, an AI does an acceptable job, and they are free, for now.