is it feature complete ? no, but to go ahead and say it's not working for 99% is just lying here is the list: https://nouveau.freedesktop.org/FeatureMatrix.html
what you can say though is that it is a bad idea for GPU that are relatively recent. wont block you from booting or displaying to screen but would probably be slow.
also this is distro dependent, I remember being given the choice with no default one (mean chosen automatically) when installing Manjaro (a derivative of arch linux).
I chose the proprietary driver since they are feature complete but later switched to nouveau driver since they are not prone to breaking on every other update. that was 2 years ago, zero problems since.
since it doesn't do CUDA either, nouveau is useless to me. next Linux build I'll either use Intel or AMD, but can't economically do ML on AMD still.
It isn't that hard to write a manual. It isn't that hard to respect user freedom to use tge device they purchased. It isn't that hard to just leave well enough alone.
But no.... Nvidia just couldn't.
Meanwhile the company responsible for the biggest security issues in the last decade still provides its CPU microcode as signed binary blob and the FLOS community is fine with that because someone at the FSF drew a random line to stand on. I will accept the FLOS communities hatred of NVIDIA the moment it stops bending over backward for the company that brought us Meltdown, Spectre, RowHammer (who needs consumer grade ECC anyway), etc. .
To this day I cannot think of a good reason to ship nouveau outside of "lets make users with nvidia cards suffer". I get the idea behind the nouveau project, but pushing an experimental driver that cannot work with 99% of the hardware it latches onto and forces users to disable it if they want a basic working system is actively malicious.