Mesa drivers are far from fine, they only got rid of the "core only" braindead schism just last year (IIRC) and that was thanks to pressure from AMD to get games working. Even then a lot of Mesa's codepaths are downright awful.
Nvidia had OpenGL as a major focus for decades (it was even the first API to get new features introduced in their hardware via extensions - though nowadays they also seem to do the same with Vulkan) and their OpenGL implementation's quality shows.
Yes, Mesa is not perfect. (particularly annoying thing is that it is possible to lock up entire GPU by invalid memory accesses in shader, infinite loops not always recover cleanly and lock up GPU too, and these bugs are accessible through WebGL, etc.)
But I prefer occasional crash in badly behaved software than abysmal performance of their proprietary implementation. (and it crashes too, usually on software using legacy pre-GL3 context).
And as to compatibility context, IMO OpenGL shouldn't have defined it at all. It is rather weird to have rendering code using both legacy fixed pipeline and modern shaders. From my experience it just causes problems everywhere, and I had encountered unexpectedly bad performance and glitches on Nvidia drivers too. (though maybe less often than on Intel/AMD)
Nvidia had OpenGL as a major focus for decades (it was even the first API to get new features introduced in their hardware via extensions - though nowadays they also seem to do the same with Vulkan) and their OpenGL implementation's quality shows.