Exactly: The OS’s were not bootable with the radeon driver. At least with it disabled kde desktop was available.
Reading from other bug reports all functionality was restored with the following line in the boot loader parameters,
modprobe -r radeon radeon.cik_support=0 radeon.si_support=0 amdgpu.cik_support=1 amdgpu.si_support=1
cpu and gpu references were not given as the failing was more generic than relevant to specific versions.