Not quite true. When UF is used to run low voltage wiring supplied
through a laboratory listed power limited supply it can be run at quite
a shallow depth of bury. That's the problem with these types of
questions. We have to assume so much to answer them.
--
Tom Horne

It still fails the common sense test, a few years from now someone will
want to put in a flower bed and destroy the entire install with one
pass with a rototiller. Granted no harm will come to them if it is low
voltage, but some folks actually like to have installed stuff stay
installed.