That's not quite how ubsershaders work. They're a "fallback" shader in the sense that they rather inefficiently implement the entire pipeline, but they do implement the entire pipeline. The shader being compiled in another thread will be more efficient as it uses only the logic needed for whatever configuration the game is calling up. But the visual result is identical in the ubsershaders case, that's the whole point. If you want, and your host system is powerful enough, you can turn ubsershaders on all the time and disable the entire compilation thread and associated cache.

I believe the term was coined by Dolphin team, who did a pretty good high level writeup of the feature here:

https://dolphin-emu.org/blog/2017/07/30/ubershaders/

It predates dolphin's use of it, though don't ask me by how long. Here's a reference to the term from 2008: https://realtimecollisiondetection.net/blog/?p=73