It's a small thing, but UUIDs are absolutely not memorizable by humans; they have zero semantic value. Sequential numeric identifiers are generally easier to transpose and the value gives some clues to its age (of course, in security contexts this can be a downside).
I take the above as a definite plus. Spent too much of my life correcting others’ use of “remembered” id’s that just happened to perfectly match the wrong thing.
Performance-wise, UUIDS are absolutely horrible for data at scale as Tom rightly points out. Everything is randomized, just awful. There are some alternate implementations of UUID that mitigate this but I've never seen them used in the wild in actual code.
That b-tree’s have been optimized to handle serial ints might be a considered a reaction to that popular (and distasteful) choice. Perhaps there should be a ’non-optimized’ option.
