There are multiple contributing factors, in addition to the fact that under hyperindividualistic late capitalism, a board entitled "Am I Being Unreasonable?" is just begging for contrarian responses. There's also substantial trolling on here, as on any popular site where it's very easy to sign up anonymously. Mumsnet is perhaps especially vulnerable because (1) it's so easy to change usernames, and (2) the moderation system is mainly reactive, relying on users reporting a post before it gets reviewed and removed.
Mumsnet is also routinely targeted by groups of incels/misogynists/MRAs who don't like women talking amongst themselves or being honest or speaking up for their own interests or really doing anything else except what they're told. (That sounds bonkers itself, but the bros often helpfully document their trolling by posting on Twitter or Reddit about it like they expect to get a medal pinned to their dick). And a non-trivial portion of the political posts are indirectly commissioned by a boss whose name rhymes with gluten.
Some posters are just taking advantage of the online anonymity to vent and rage, often about things only very tangentially related to the thread. Some are trying to help but project their own specific experiences onto a situation that may not be similar - your husband's definitely having an affair because mine also switched from Builder's to PG Tips and six months later ran away with his riding instructor. And yes, some people probably do have genuine MH challenges, and/or may not be completely sober. I don't agree with the person who said upthread that you'll never get usable advice here. I've seen plenty, but you may have to triage and skip over any replies that are irrelevant to your question or situation.
Obviously I wouldn’t admit to using mumsnet in real life! Indeed, there have been reports here from people who have got into terrible trouble over this - once a group of English tourists on holiday to Scotland reported being followed everywhere by neds yelling "yer mum's on Mumsnet!"
Being a genuinely kind person ... This is a more difficult problem. I recommend Victoria Smith's new book, [Un]Kind: How 'Be Kind' Entrenches Sexism.