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)
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)