They dont have any open source drivers for ANY platform. So that's the same on Windows and Mac. On Linux you have the Nouveau (open source, independent) drivers for nVidia though, and you can already play some games with it despite major performance loss.
As for graphics switching you'll be happy to know this is a problem that the SOLUS distro is going to tackle in 2017 to have a better solution than the existing ones which are mostly broken.
I have deployed thousands of workstations with various ranges of graphics cards. Nvidia is the simplest, a trailing second is AMD (they do make rolling RPMs easy though) They are just as buggy as the windows drivers.
Intel make brilliant drivers now, its just a shame the graphics cards are tiny compared to AMD/Nvidia.
Nvidia just don't do opensource drivers. For me, I frankly couldn't give a fig if they are open source or not, just so long as it works.
If NVidia, Intel, and AMD got serious about Linux graphics support for enthusiasts and professionals, I'm sure we'd see a huge improvement and make switching viable for many traditional hold-outs. Chicken and egg, unfortunately.