I can confirm Sergio's findings so far, though kernel-4.15.0-0.rc4.git4.1.fc28.x86_64 (based on git commit ead68f216110) hangs completely upon switching fbcon to nouveaufb if I have "rhgb" in the kernel command line. I won't be able to provide more details for some time as I have to give back the machine where this can be reproduced. I'll keep my fingers crossed for Sergio.