A lot to unpack. A few comments for now, maybe more later:
It’s not necessarily negative to not make good food.
It is negative for a restaurant to not make "good food" - whatever that is. It is not objectively negative for stam a person to not make good food, depending on the context, though it often is still LH. That is: not making good food is the same as any other talent assessment, of which a negative report is bad. So when it is nogea to A) a person's opinion of another, regardless of how ridiculous it is, or B) a person's financial situation because he is trying to make money on the basis of that talent - it is LH to say that he lacks that talent. B is obviously nogea when it comes to claiming a restaurant doesn't make good food. A is nogea also (even for someone who will never eat there) because it reflects badly on a person to say he has opened a business in food but has no talent in it. Lechora.
- Rule #3 requires that "One must be motivated by the need to help others avoid the pain that one experienced, and not motivated by a desire for revenge." I can't speak for others, but personally, if I'm 100% honest with myself, when I've felt like leaving a bad review for a business, more often than not it was at least a bit motivated by a desire to get back after feeling like I was ripped off by a bad experience. Even if there would be a toeles, I'm pretty sure that disqualifies me from saying it, even if it would also help others.
As noted after, Rabbi Berkovits's take is this: bad motivation automatically disqualifies LH Ltoeles only where lo saamod does not apply (but rather, one of the other heterim). Where lo saamod does apply, he implied that there are some cases where MIXED motivation (you don't like the guy AND you don't want others to go through your experience) is enough to matir - though noted that your accuracy is SUSPECT if you don't like the guy. Lastly, in some cases, he said that you have to get over your personal negative feelings and only speak lesheim shomayim because lo saamod doesn't let you off the hook. I'm afraid it isn't clear to me which psak applies in which situation, and conversations with him and various talmidim have not cleared it up.
Comment apropos to nothing: there is a huge difference between posting a review listing pros and cons versus an entirely negative review. The former has an umdena that the owner wants the publicity.
Some have commented regarding the fact that experiences are not always the same. (For example, one time it was overdone, the other time it was fine.) The Rav has said that it is fine to say this - after all, if it happened once, it can happen again, and one is permitted to let the reader know that this is within the realm of possibility. However, it should be expressed that way ("This time, the food was overcooked. I don't have enough experience to know if this is always the way that they make it.") If there are extenuating circumstances, one MUST say so. ("They were busier than they usually are.") If there may have been extenuating circumstances, one MUST express the possibility. If a mistake happened, you can tell people it happened unless it is unlikely in your opinion that it would reoccur.
However, the above applies to a mixed review, not a negative review. Once there is an umdena that the owner would not want the review written, none of these heterim apply without the rules of toeles being fully accounted for (including an attempt to complain in private, for example).
He didn't say so, but I suspect a mixed review is actually muttar for anyone to read, but a negative review shouldn't be read without the reader expecting toeles. (And can you know which a review is in advance?) This is why I personally do not read them until I know I will need to read them.
And because no one has mentioned it iirc - it is ossur to believe the reviews you read, only to take them into account without believing them. Just saying...