It's better to use Debian Unstable because Testing doesn't get timely security updates.

Unstable (aka Sid) rarely lives up to its name, having ran it as my daily driver for a few years. Comparatively, the months I spent on Arch were marred by packaging issues resulting in me switching back to Debian.

I want the maintainer to hold back an update and write patches when there are noteworthy regressions that hurt my workflow. The Debian package maintainers consistently act in my best interest, not that of the developer's or outside political interests. Turns out I find that really useful.

Debian Unstable is the base for Ubuntu.

It's not unstable at all.

The "unstable" there means that everything changes all the time. It does not mean that things break (although they do break way more often than stable - when I tried it, I did have to reinstall the OS every few years).

I was under the impression that `unstable` is... well, not stable (that it breaks). Is it usable for someone who doesn't want to spend much time fixing the system?

In Debian 'unstable' means that the package versions installed aren't stable. That is, there is a high level of package churn just like any rolling release. It isn't any more buggy/crash prone than any other rolling release. There is just a risk at running bleeding edge vs. vetted/known working software.

Docker makes the base OS largely irrelevant for testing in my workflows.