Except for the "reasonable rate" bit, which (IMHO) also includes "reasonable latency." Real UARTs can be used for, e.g., pulse-per-second inputs from GPS or atomic clocks to discipline an NTP server. That doesn't work for a USB->serial thingy (because the latency is literally three orders of magnitude worse).

It doesn't always work.

I've seen serial devices that are so picky with timing that they'll work with real serial ports but most USB->Serial adapters cause programming to fail.