It's O.K. to criticize commercial products. It's not O.K. to criticize people or their cars. If you have a product performance claim or criticism, use factual evidence. Dyno tests are the holy grail, but most of us don't have that option. Published tests of products in reputable magazines are good too.
Rumors are even O.K., as long as they are identified as such.
Don't take product criticisms personally, even if you designed or sold the product. The critic is usually just trying to be helpful and their factual criticisms deserve an honest response. Keep your language, jokes and allusions clean . Not everybody shares the same sense of humor and there are plenty of other places to practice your favorite jokes, analogies, whatever. If you wouldn't share it with your mother, you shouldn't share it with us.