I don't agree with your first point at all, posting on other platforms is trivial and there's what must be hundreds of post scheduling options you can hook into an RSS feed. Here's a completely open source one that you have to spend a lot of time configuring API keys for, but then it just works: https://postiz.com/

What makes it difficult is all of the quirks you have to account for. For the most trivial example, Twitter has a character limit of 280 characters, but it's 300 on Bluesky, 500 on Threads, and on Mastodon it is whatever your instance wants it to be.

For another quirk, I have like a side-project in which I publish snippets of DJs playing copyrighted music, and while I can post those videos on TikTok/Instagram/YouTube without worrying about copyright, I am like 99% confident my website would be instantly delisted from all the search engines if I used the POSSE strategy for that use case.

I agree with your second point that getting anything useful out of it (as in traffic to the source) is pretty much impossible. On Instagram you can only do that via stories, but you can't automate it, because you need Instagram's story editor to add a link to the story. On TikTok you can't even put a link-in-bio until you reach a 1000 followers. On Twitter you might as well not bother, as the medium itself prefers completely unsourced claims. As for Facebook, I honestly don't even know why anyone would bother with Facebook these days, it's completely irrelevant.