There are those who are accused of trolling by doing exactly this, although from their point of view they are enlightening potential customers to the shortcomings of the product.
Nothing wrong whit that for example AmigaONE-XE/SE has hardware defects is true.
But to say Pegasus II, Sam series or X1000 is buggy is not true, they are good computers.
But if your trying to sell MorphOS to some one that was initial interested in AmigaOS4, then there is some thing wrong with that.
The solution to the problem does not lie in some written rule, it lies in softening attitudes and reducing sensitivities to criticism.
Criticism in it self is not wrong, it depends on how its used, some criticism can be good to improve things, user feedback often good criticism.
None user criticism that is incorrect and is repeated to spread FUD is not.
What derails the thread: the single cirtical post, the dozens of over-sensitive reactions, or the combination of the two?
That's way people should stay on topic in the first place.