It's called defamation and you can be sued for it, regardless of whether or not your review is truthfull. Thais are very insecure and to be called out wrong they worry they will lose face. It's very rare that a Thai will admit they are wrong.
This gets very confusing. When I return to Thailand from another country do I have to complete a TM30? I live with my Thai wife in a rental property in her name.