It’s also a fscking mess to set up a Usenet downloader, especially since it’d be a bunch of buggy weird stuff ending with -arr in the names and web UIs.

It’s really not. You pretty much need to just put in some api keys for your indexer, downloader, and provider, and away you go.

In Usenet messages are replicated over all services offering that newsgroup. I hope the downsides are clear.

What downsides are you talking about in regards to downloading content from usenet?