What's available on Ubuntu that isn't available on macOS?
Lots of open-source stuff that is technically available on macOS but not practically. Partially because Ubuntu's package manager is better. Currently I run all that kind of stuff in a headless Linux VM on my Mac. Trying to deal with any of it in macOS is a waste of time.
If you care about video games (I don't), Ubuntu is also better just because of Steam Proton. But since that's CPU architecture specific, it's not as relevant in this discussion. Linux also has broader GPU support, maybe relevant for eGPUs or the coming Mac Pro.
As a daily usage OS, Ubuntu is awful. The GUI is jank. The DE uses more power than the Death Star. Random websites just won't work. Waste of time unless you're doing it for the free software movement, in which case good for you.