IMO the killer feature of Medium/Substack is that they've already built up an audience willing to read long-form content, and they're happy to immediately start

IMO the killer feature of Medium/Substack is that they've already built up an audience willing to read long-form content, and they're happy to immediately start recommending your content to interested sections of that audience. Search engines too are happy to surface content from those sites. Self-hosting is ok if you're not writing for an audience, or the legwork of promoting yourself, building up an audience from scratch, and doing SEO doesn't put you off.