It’s definitely possible to make a really stellar experience, but that winds up being the exception. The URL and history state are sort of “invisible” elements of the user experience but require thoughtful care and attention to what the user expects/wants at each step, a level of attention which is already a rarity in web development even in the most visible parts of a page…so frequently the history/back button stuff just totally sucks.

Yeah, in my experience you only get great stuff when both product and engineering has equal care for the final experience. If either parties lack care, you'll miss stuff, particularly things that are "invisible" as you say.