I think the value proposition they're offering is a carefully integrated system where everything has been thoroughly engineered/tested to work with everything else, down to writing custom firmware to guarantee that it's all ship-shape, so that customers don't have to touch any of the innards, and will probably just treat them as a black box. It seems like it's chock-full of stuff that they custom-built and that nobody else would be familiar with, by design. If that's not what you want, this probably isn't the product for you.

I can somewhat see your point, but in my experience you can't rely on RHEL or whatever vendor Linux to correctly bring up random OEM hardware. You will slowly discover all of the quirks, like it didn't initialize the platform EDAC the way you expected, or it didn't resolve some weird IRQ issue, etc. Nothing about my experience leads me to believe Linux will JFW on a given box, so I don't feel like Linux has an advantage in this regard, or that niche operating systems have a disadvantage. Certainly I feel like a first-party OS from the hardware vendor is going to have a lot of advantages.