I do not know why. I do not see how the boot loader would have any effect on a system after booting. If "can't start xorg" means the GUI does not load and the system returns to the TTY, create a file
/etc/sysctl.conf
and add machdep.allowaperture=1
Some hardware require that addition for the kernel to access drivers. Again, I do not understand how the boot loader could affect a running system, but I am not an expert. So it is worth a try.
I have never used uefi boot and so have never had a need to research the matter. All I can say is that many people do use uefi with OpenBSD.