Unfortunately, all these share the same problem for XF: there's no way for any of these to control XenForo in any form other than the most rudimentary or hacky workarounds. For example, I'd love to set up an agile, one-click-deployable system for rolling out add-on updates to my site as I write them. I'd love to commit to master, and have Jenkins or something similar update the add-on repository on the server, then upgrade from the XML.
In my opinion, the best and most accessible way to manipulate the XenForo install - whether this be part of a full-scale API (ideally) or a separate feature - would be to provide a CLI-bsaed endpoint, in a very similar fashion to how the CLI installer/upgrader currently works.
For example, to achieve my earlier use case, I could set up my tools to call php -f xenforo/library/cli.php --upgrade-addon xenforo/library/MyCoolAddon/addon.xml, and have it go through the process of upgrading and rebuilding in the CLI.
A non-exhaustive list of things I think this should be able to do are:
- Install, uninstall, upgrade add-ons
- Enable and disable add-ons
- Move the site in and out of maintenance mode (with definable reason)
- Rebuild caches
- Trigger cron jobs
- Display statistics regarding the XenForo installation - most importantly, the XF version, but perhaps the PHP or MySQL versions too, or the numbers of users/discussions/posts.