How bad is it for a Mac to forcefully restart it when it freezes?
Holding the power button and letting the System Management Controller SMC perform an immediate halt is no worse than letting the software initiate the halt.

I suppose you could come up with a scenario with the mac in free fall or something relating to the head not being parked, but modern hardware is designed to handle power losses from the wall outlet with great grace and here the SMC is still controlling a soft landing.

The power implications are the same even though the applications and operating system don't get their normal opportunity to save work, close files and get ready for the next time they start.

Your files and data could be at risk from this (think erase and install OS and restore backup files as worst software case), but the hardware doesn't care of the open files or some pending changes are not flushed from caches before the lights go out.