Sometimes there are regressions in the kernel and other driver issues. My laptop is more than five years old and I had to boot an older kernel for a while until a regression got fixed. I guess that's less likely to happen on Windows. Not being as close to the hardware vendors means there are bound to be more edge cases even on boring distros like Ubuntu.
Someone might say I can install Linux on my PC but then I have to deal with maintaining it.
So, what I hope the Steam Machine is, is effectively PC based console with no Microsoft, no root kits, and no maintainence.