This feature just isn't ready for prime time.
I spend like 99% of time at my house, in a location with ample wireless and cellular signal, and yet it's not considered a "Familiar Location" by Apple.
Today, I had to reset my passcode, which required me to wait an hour. On a lark, I opened a chat case with Apple. The chat technician told me that, because I wasn't at home when I set up my phone, my home is not considered a Familiar Location.
I politely informed him that, first of all, I was at home when I set up my phone. Secondly, I told him, the Apple Support page (About Stolen Device Protection for iPhone - Apple Support) states that Familiar Locations "typically include your home, work, and certain other locations where you regularly use your iPhone" and that there's no indication that it's related to the location where you set up your phone.
He thanked me for checking the support page, and then escalated to a support technician on a call. This technician informed me that the entire Familiar Locations feature is automated and there's no way to modify it.
So, it's an automated feature in a black box, but it doesn't actually work. Which is just another way of saying that it's broken and probably not worth enabling. Bummer.