Abe let your HTML pure: Nothing is added to your templates. The Sitemap plugin helps you inform the search crawlers about your pages. Free the SEO !
Just configure the deployment you need, choose between amazon S3, Surge, Github, or any (S)FTP reachable server, and you're good to go !
Creating your user database and a custom validation workflow ? It's one json away. Give rights en permissions to your users in a blink! AbeCMS is using oAuth2 (of course)
And as your needs are specific, you can use existing plugins from others or create your owns. Algolia, Asana, Google Maps, PM2... Yours... What else ?