Is it safe to force shutdown a mac while booting?
Any forced shutdown can leave a file in a bad state. At boot, you would only corrupt a system file, so that's usually easy to reinstall the OS so I suppose if you care about user data - that's the best time to force a stop.

High Sierra supports internet recovery and bootable install media, so the options to reinstall are much better than the old days where you needed a CD or DVD to reinstall and it took a long time.