Not exactly a product from ublue but something in the same line:
Secureblue because of the reasons aforementioned for the ublue images where things are really darn rock solid out of the box AND because Linux is fundamentally behind in security and this project is trying to mitigate some of the big flaws.
Some answers to your first question you can find here: https://madaidans-insecurities.github.io/guides/linux-hardening.html
For the second question about in what ways Secureblue do mitigate that you can find more here: https://secureblue.dev/features
The last question about usability, is very usable. If you use Bazzite you may have a similar experience. It is not like QubesOS that isolate all processes making it even not able to use a GPU.