It's not even necessarily it being strictly monotonic. That part does help though as you don't need to skip rows.

For me the bigger thing is the randomness. A uid being random for a given row means the opposite is true; any given index entry points to a completely random heap entry.

When backfilling this leads to massive write amplification. Consider a table with rows taking up 40 bytes, so roughly 200 entries per page. If I backfill 1k rows sorted by the id then under normal circumstances I'd expect to update 6-7 pages which is ~50kiB of heap writes.

Whereas if I do that sort of backfill with a uid then I'd expect to encounter each page on a separate row. That means 1k rows backfilled is going to be around 8MB of writes to the heap.