You cannot get sued for a review just because it's lower than 5 stars. But of course if you write something like "I found a dead cockroach in my pizza", or "I hard that they don't clean the dishes enough" in a review without proof, that's defamation. And it doesn't matter if you give 1 or 5 stars with the review.
Also competitors would leave shit reviews and Googles completely brick walling any way to rectify or moderate reviews made it a shit service in true Google fashion. Google and service live in two different worlds.
I wrote a one star review and posted pictures of a badly burnt pizza served at a restaurant in Berlin. Google sent me an email telling me they removed that because the restaurant filed a defamation claim
Multiple people report that comments such as "Service was slow", or three star rating without comment were removed.
If Google started to show how many reviews were removed, it was because the lawfare started to seriously affect the ratings.