It's easest to understand how to use them where the items are ones you'd just substitute without much thinking.
For example bolts of a certain thread, but different lengths.
The more complicated they are, the more one has to pay attention.
One sneaky result of using FTs is that if the name you refer to doesn't exist in the FT then it will quietly substitute the generic, maybe toss a warning along with the other hundred or so going by so it isn't noticeable. Make sure the generic is something noteworthy.
Another thing is that if a feature would make the FT instance fail, it will be quietly suppressed. That's another fun one to debug.
Still, I can't say I've had a lot of trouble with them, but when they are made by users with little programming background who don't realize that Family means Family, and that they all are related, it can become an interesting knot to untangle.
I should write a book, but who would buy it? Besides, PTC developers should write the book.