> I suppose RHEL uses basically the same kernel, with patches that don't alter its interface too substantially. So I presume that a driver in source form, or even partly in binary blob form, should build and work approximately equally well with any stock kernel.

No, I think that's exactly the difference; Linux (in)famously has no stable ABI for drivers, and regularly makes changes that break things if you don't recompile against the updated kernel. Part of the value in RHEL was that they froze significantly more ABI surface, which made it possible to write proprietary drivers that would work across at least minor kernel updates without needing to be changed. That won't work on an upstream kernel because upstream doesn't go out of their way to freeze that ABI surface.