I think that the important heuristic we should apply to rants to decide what flag to use should be:
Would I flag it as rude or abusive even if I didn't disagree with the opinion it expresses?
If the answer is "yes," then we are likely using that flag correctly. Otherwise it's just not an answer.
People sometimes rant about how Ubuntu is great or--more often--how some other OS is bad. I don't think I've ever seen such a rant flagged "rude or abusive" unless there was something else wrong with it besides being a rant.
It seems to me that custom mod flags should not be used for rants (or only in very rare cases, if ever). However, I understand why users may have been using them recently for this purpose--even though I don't think our policy should be to use them--because currently our policy is ambiguous, and when no other flag is clearly correct, a custom flag is generally considered reasonable. Moving forward, however, we should not advise users to custom-flag rants.
What I mean when I say that our current policy is ambiguous is this:
Those official guidelines--which were recently featured (by you)--would have us not flag rants as rude or abusive just for being rants.
Still, that does not prove me right, because that answer needs updating. As I understand it, we unambiguously consider posts that deliberately consist of complete nonsense--like, not even words, or a repeated word, or words strung together in a truly random fashion--to qualify as "rude or abusive" on the grounds that they are an abuse of the system. That post should at least be updated to cover that.
The reason I don't think that usually applies to mere rants is that while rudeness and abusiveness can sometimes be somewhat gauged by our own reactions, as in the case of personal attacks or expressions of bigotry and hatred against human beings, abuse of the system is impersonal and should have nothing to do with our personal feelings about the content of the rant. But in practice I strongly believe it does, always has, and always will.
But this other upvoted answer suggests (as you have rightly pointed out) that many rants may be considered "rude or abusive."
Actually, I do not think even that supports flagging all rants, because "with a bad intention" means a deliberate misuse of a system. But many rants are by new users who don't really know the rules, and whom we're required to be patient with.
As I alluded to above, rants can be rude or abusive for a number of reasons. They could insult specific individuals, they could be vehicles for bigotry. But if we're going to flag rants on the grounds that they are not appropriate for respectful discourse, then we have to wrestle with the problem that ranty expositions are considered socially appropriate in numerous contexts and the real reason we don't want them as questions or answers is that they're not questions or answers, which we already have flags for, such as the not an answer flag.
Becoming upset about the software one is using is a common experience. So is using software incorrectly. That includes Stack Exchange. We close numerous questions and delete numerous answers very soon after they are posted, and in most cases we are right to do so--especially for answers that aren't actually answers. This is a normal part of the functioning of Stack Exchange, in part because we are so different from a normal forum. And yes, people should take the tour and read the help, but failing to read the documentation is extremely common, too.
That a post's author is deliberately doing the wrong thing is far from the only reason we might raise a "rude or abusive" (or "spam") flag on their post. For example, a post that belittles another user, or that advocates systematic violence against an ethnic group, is a big problem for our community irrespective of the author's intentions. But a rant about Ubuntu does not rise to that level. It does not get anywhere close to that level.
So the question that remains is: Is this post deliberately misusing the system in a serious way? I think we often cannot say "yes" to that with any certainty. I don't think we want to blur the line between (a) what needs to be deleted soon and (b) what needs to be deleted immediately and the author marked in the system as someone who may be a threat to our community. That's what the "rude or abusive" flag does. And even if we did want to blur that line, I don't think guideline #2 in the "Be nice" policy permits us to do so.