Mike, Can’t speak for the ULA packs, but I’ve read and am just passing on the following info: many (not all) packs have fairly water resistant material and could survive some rain IF any seams present on a pack were sealed.
I do know first hand that, If I don’t think the rain is heavy enough to done a poncho or cape, I have found water entry it seems (no pun intended) along any seams in the pack.
That said, I have never tried to seam seal a pack. I usually just don the poncho or cape or if the rain is light just hope that the WildThings Epic fabric windshirt and often LLBean Epic fabric trail cargo pants) I often wear will be sufficient.
Please note that this Post is just intended to be a general FYI and is definitely NOT a criticism of ULA and by all knowledgeable accounts its superb looking and immaculately crafted packs or any other Mfr’s packs