Actually your second paragraph ("I hate the concept") wasn't bad, but when you pile on complaints about "the website is garbage" and "I hate the cringeworthy copy", it turns into more of a beating, which is not how we want people to relate to each other here. It also undermines your claim to principled criticism; it is as if you were disagreeing with someone about an ethical issue and then started criticizing how they dress and the fact that their car needs washing. I'd say that's where the line gets crossed. If use a hammer to hit a nail, that's not fulmination, even if you hit it hard. But if you hit the nail and then start hammering the door and the floor and the wall as well, that's more like fulmination.

You certainly don't have to be unemotional! but internet rants where people just try to destroy each other (or their work) have a distinctive quality where the emotion is out of sync with the underlying topic, as if one is venting in relation to something else that remains unsaid—for example, the personal experience that the emotion is related to. What makes emotion meaningful to others is when we share personal context of the emotion—then we're sharing something of ourselves. But if you only express the force of the emotion and hide the personal context, this has a disconnecting quality. It also easily starts to resemble aggression.

There are several reasons why this is bad and damages the container here. The most important is that readers can feel this out-of-syncness and excess force, even if the commenter isn't aware of it. Readers who (for whatever reason) identify with the opposite side of the question will feel it as a provocation, and take it as a license to vent their own excess force as well. Actually, since we all overestimate what others do and underestimate what we ourselves do (by at least 10x), they will likely respond even worse. This is how we get destructive flamewars.