We're probably a lot closer in viewpoint than our posts seem to indicate. Sure, verifying employment tells you nothing assuming the candidate didn't lie on the resume. However, and this is where we seem to differ, first and last day are easily verified facts. Giving that information doesn't say anything one way or the other about an employee's performance and I suspect a lawsuit alleging defamation because "only giving employment dates may be taken as I was a bad employee" would get laughed out of court.
Finally, the examples you give are subjective and could lead to a more reasonable basis for suing; even if they wouldn't win.Take "not fitting in with corporate culture." While they may hold that viewpoint saying that about an employee is making a statement, purported to be factual, about the employee; a statement whose accuracy could be reasonably questioned. Maybe the person giving the reference was just a bad boss who didn't fit in and drove employees out.
Even the example where you claim to have hard numbers, such as time late may be questionable. Maybe they took sick leave and you didn't realize it. You may have talked to them about it and they gave valid reasons that you didn't accept. Or, maybe you let others come in late and didn't do anything about it because you had it in for this employee.
Sure, anyone can sue over anything, but the examples you gave could easily be construed as not factual and this potentially defamatory.
In the end, we may just have to agree to disagree.