Don’t get me wrong: Flatpress will run on the NGINX web server, like the one on this tilde.club machine. But you give up the PrettyURLs plugin, which requires an .htaccess file that is used with the Apache web server. So you end up with not-so-pretty URLs like https://tilde.club/~ … y:entry231205-120940. You still have a working blog (and I kind of admire the chutzpah of WordPress users who default to numerical-entry URLs on their sites).
The other thing that doesn’t work is the Accessible Antispam plugin. I’m not sure if that also relies on Apache, but on my site it just asks you to solve math problems in a neverending loop without accepting your comment.