The Piguet 21 has been indeed used in many watches for a very long time, among them many Cartier and Blancpain watches. I've never heard of any lack of reliability.
Is it a relatively easy watch to service, find parts, etc.? I love the cleanliness of the dial and am hoping the movement matches in simplicity (and durability). How would you compare it to the JLC Ultra Thin cal 849?