Force Shutdown: How Harmful?

Not very harmful - thanks to journaling: Mac OS X: About file system journaling - Apple Support

I've forced-rebooted quite a lot in my beta-testing days, and have never noticed any side-effects, aside from loss of data when I forgot to save a file. I wouldn't make a habit of it though.

Perhaps the more important question you should be asking is the need for the forced shutdown.

Ciao.

... Out of curiosity, how harmful is a force shutdown for MacBooks/Apple products?

It's not, since the only resulting effect is to cause a cursory file system integrity verification to be performed upon the subsequent boot. That results in an insignificant amount of additional disk read/write activity.

If you have not already done so, I recommend that you implement a reliable backup strategy such as Time Machine: Use Time Machine to back up or restore your Mac - Apple Support. That recommendation applies regardless of the need to perform the forced shutdown or any other abnormality.

You didn't explain the need to do that though. It should not be necessary, and could be a symptom of an underlying hardware problem. Or, it may indicate nothing at all. I would not be concerned unless the need to force a shutdown becomes increasingly frequent. In that event you might be able to correlate it to the actions that resulted in the need for an abnormal shutdown.

Apple tells you how to do a forced shutdown - because they have never fixed problems with frozen desk tops. Its a rare event when the function keys to force quit or force a restart actually work.

There is a console log that does list all of your invalid shutdowns.

It may harm the index for a Time Machine drive - where you cannot enter the time machine drive without getting stuck and having to force book again.

In my opinion the freezes are an operating system software error where a memory leak causes a gpu error that is totally bogus.