That sounds doubtful to me. I'm not a lawyer (and I don't think it sounds like you are either?) but at least in the US, my understanding is that courts grant quite a bit of latitude to people speaking freely about their own experiences. I don't really know though, but I would definitely be surprised (and incensed) to find out that this is a widespread problem.
I wouldn't be surprised at all, and at the end of the day you can sue for anything, even if you don't believe you can win. Mounting a successful defense costs money, even if the claims are without merit.
Oh sure, you can sue for anything, and defending yourself is expensive. This is a problem in general, but not one worth worrying about in practice (will I get sued for writing this comment - I could!), but I'm a lot more interested in this part of your comment:
> and possibly even win, given that it's probably hard to back up a lot of these accusations in a way that would hold up in court
Assuming I didn't sign any explicit agreement (like as part of a settlement), if I say, "I did not find X to be a good employer because of Y", I do not believe it is necessary to prove Y in court in order to win a defamation suit.
But we're both guessing. I would say that your side would be easier to find evidence of, though, if you're right: do you know of, or can you find, examples of this sort of suit being brought and won?
