Community Builder can be upgraded through Joomla native upgrader, but only for Stable releases at this time, not for nightly builds.

Not sure to understand your question. Can you refine it please ?

Would you like also nightly Community Builder releases to appear in Joomla's updater ?

That's something that we have in advanced development, but current issue is that Joomla's upgrader doesn't fully support SemVer versions extending numbering scheme that we use for nightly builds.

That is an interesting idea.

Until now, add-ons were requiring being logged-into www.Joomlapolis.com to update them.

Now that we introduced site keys a couple of weeks back, that is not required anymore, and it would allow to check for updates of CB Add-ons in the same request as the CB check, and even to deliver all add-ons in the update.

It would probably imply to have the new updates-checker of CB (not yet activated) being able to also check for CB add-ons and the corresponding updates package delivery to (also) include add-ons needing updating.

I have written down a corresponding Feature Proposal here:
forge.joomlapolis.com/issues/8086

Such an important change needs quite some coding and would require extensive testing, as potentially extensive site updates must work in a variety of conditions, so it's probably not something to expect soon.

But we plan to add Add-ons updates notifications with an update button in the same notification as for CB update in the Joomla dashboard soon, which would already solve half of the problem (being notified and be able to do a one-click update).