I'd recommend coating them with triple thick as soon as possible to prevent any future flaking. The Stern backglasses from the late 70s and early 80s, in my opinion, are some of the best pinball artwork and are definitely worth saving.
I'd say stern backglasses are no different in quality compared to any other Bally or Williams from the same time, the major contributor to cracking and flaking over time is how the machine has been stored over time.