Also, in most countries (especially the US), also a defamation problem.

In the US? How is it different than yelp? (Except for the extortion)

There's a difference between "I didn't like their salad" (opinion) and "they don't pay their bills" (a fact, that could be defamation if not true)

	▲	microgpt 8 hours ago \| parent \| next [-]
		In Germany if you leave a Google review saying "the salad tasted bad" you may get sued for defamation, but probably not because they'll threaten Google first, who will take it down and ding your account. If they do sue you, they will win unless you can prove to a judge that the salad tasted bad. You will then have to pay a few thousand dollars of damages and another few thousand of legal fees (yours and theirs).
	▲	lokar 7 hours ago \| parent \| prev [-]
		What about “the service was slow”