And NI are absolutely writing kernel-level drivers, since they in addition to software for measurement and automation they offers a huge number of different hardware. So to support all their measurement, dataloging and automation hardware, they have to write drivers for PXI (PCI eXtensions for Instrumentation), VXI, PXI- and PCI-based modular instruments and GPIB bus controllers. Their Linux offering have been rapidly growing for years, so obviously their userbase already have a strong interest in Linux.
Even if I disagree in that NI's hardware are considerably less complex than the hardware of NVidia and AT. For some I think actually the opposite are true, but it does not matter. What makes it hard for NI are simply the huge amount of different hardware they have to support, most likely many more than any other hardware vendor doing Linux support. Combined with the fact that their userbase expect support for far longer time than you can expect from the likes of nVidia or ATI. And very few, if any, of the regular kernel developers have or use any of their rather specialized hardware, makes it rather inconvenient and expensive when the kernel API changes. IMHO, the only strong reason for a stable kernel API are vendors like NI making specailzed, non-mainstream hardware.
