HFS+ is a journalled filesystem, so forced restarts are not such a big problem. What does a journalled filesystem do? It essentially ensures that any change made to the filesystem is "atomic", which means that it either entirely completes or does not happen at all. If you force-restart while the disk is in the middle of an operation then the next time the computer starts up it will look at the filesystem's journal and detect that an incomplete operation occurred. It will then undo the incomplete change. (Note that this is how a journalled filesystem *could* work in principle, not how HFS+ actually works AFAIK.)
In other words, it's much harder to corrupt a journalled filesystem even if you force-restart or lose power.