> Will homebrew work on Ubuntu?

Yes, brew works well on any x86 Linux distro (doesn't support ARM yet, sadly).

> Why is there no Debian or Ubuntu package or ppa listed?

Packaging for every distro is a lot of work, and the Nu core team has had other priorities. I think we'd like to have Debian-specific packages eventually but that will probably require some work by a motivated contributor. In the meantime we publish binaries, and it's fairly easy to build from scratch or use Homebrew.