I guess you need to be prepared for some downtime with your car when going through FCP replacement warranty.
Say you purchased a water pump form FCP, and the pump fails three years down the road. Your car is on the side of the road.
How does the whole thing work? Do you have to remove the water pump from the car then ship it to them and then they send you the replacement new one. That's easy week to two weeks of downtime for me considering I am ordering from Canada.
The idea is lucrative and I'd say in favor of US citizens, but even then you would have some downtime up to a week. In that week you will need to get a rental car, deal with towing. The cost will still add up.
Edit: Some questions are answered here
https://www.fcpeuro.com/page/lifetime-guarantee