I also want to see Coreboot shipping on Thelio. To me, OOTB Coreboot has become their prime differentiator. I run their Pop! OS on my Ryzen rig, but UEFI is a nightmare.
I do suspect Coreboot is a less solid option on desktop. Aside from pretty much no desktop Zen support today, desktops tend to have more flexibility for the end user, and the minimal nature of coreboot could perhaps backfire there.
How much does coreboot care? It needs to support the specific motherboard+CPU, but after that it's not like coreboot needs to handle most hardware. Excepting of course storage (USB, SATA, etc.), but that should be equivalent to laptop difficulty, and possibly network cards for network booting, which I grant might well be harder in desktops.
But mostly I'd expect coreboot to need fairly minimal hardware support and then just pass off to the real OS with proper drivers.
Coreboot supports multiple payloads[1]. You can in fact run Coreboot without UEFI - for example by using a SeaBIOS or GRUB payload.
When you are running Coreboot, it is virtually the first thing that executes on the processor, probably after some burnt-on initialization code. At that point it can do whatever. I suspect most people using Coreboot are not using it to launch Tianocore, the UEFI payload, but I guess I don’t know.
The only logical reason I can see to need UEFI would be to load drivers and/or option ROMs that need a UEFI environment - but nothing stops you from implementing drivers/initialization yourself, reverse engineering notwithstanding.
