All discourse.org hosting that’s not enterprise runs on multisite, so it works and it’s stable and it doesn’t change much.
I think you can use discourse_connect to have one site auth for all of them.
I’ve had a couple people who wanted to create some reddit-like thing with hundreds to thousands of sites but none of them were serious and I never got beyond proof of concept.
I’ve got tooling to spin up multisite instances, as well as multiple Discourse containers behind a reverse proxy with a front end to manage them (https://dashboard.literatecomputing.com/).