> There's a big difference between "almost" and "always"

Not in the context of an RDBMS, which use b+/b*-tree variants (or LSM sstables). Sequentially generated UUID's will end up near each other when sorted lexicographically, regardless of the fact that the sort order doesn't perfectly match the timestamp order.

The key seems to be based on UUIDv7, starting with a timestamp in milliseconds.

So the order can become out of sync if multiple events happen in the same millisecond; or if your servers' clock error is greater than a millisecond (i.e. if you're an NTP user)

More than sufficient for things like ordering tweets. If you're ordering bank account transactions, well, you'd probably be using transactions in an ACID-compliant relational database.