Although I can see the rationale of not saying something terrible about somebody, for it could seem slanderous. I fail to see how you can be sued for saying something positive.
Ah well.
I think the vast majority of people believe that they are undervalued. Even if you say something positive about such persons, they will believe that you have understated the matter and will resent it.
One person who had worked for me until he was fired for incompetence and laziness, had the audacity to use me as a reference some years later, perhaps not thinking that the reference would be checked.
When informed that I had been used as a reference, my response was, "Really?". The voice on the other end of the line said, "That's all I need to know. Thanks."