Sometimes, support never comes at all, even for hardware that has been sold for years and is still selling. Take an Atom with a PowerVR GPU, or a VIA CPU with VIA GPU in the chipset. There's near zero level of support for anything from the vendors, so the community rushes basic support for 2D operation (not being stuck with slow VESA 800x600, 1024x768, 1280x1024, with no way at all to get other resolutions running). Even then, mere video playback is a crapshoot (on VIA VX900, it's not really smooth for divx playback, an ATI rage pro would be much better).
Open source 3D drivers? That only realistically gets done for the big three (Intel, AMD and NV). Even then your particular generation has to have a decent driver and be well maintained. And if your driver sucks who knows what will happen to you, a crash, lack of power management, displaying black or grey rectangles, or maybe just 40% CPU usage for minimizing a windows.