I used one site which would throw an javascript exception and break because it called part of google analytics from some other function that was needed to make the site work. I ended up opening it up in a private browsing session, and I've got my script blocker disabled there.
Actually, because of this, I tend to temporarily disable blocking at the moment of payment, because there are too many redirects and reloads happening at that time to reliably use a blocker. It partially defeats blocking, but at least they aren't tracking me during regular browsing.
The travel sites I saw blocked didn't fail because of GA blocking I believe, but they were depending on various other scripts. I believe they just had messy code that relied on callbacks from third party scripts -- some of which were blocked by uBlock.
Might be worthwhile finding a different online travel company, if they can't get this right I can't imagine what else they'll stuff up when you make a booking!
What? How does that even work? If GA goes down, their site just stops working until it's back up?