Peter Lander came up with the following;:

Don't rely on them, particularly cheap after-market kits, until you 'know'
they work well.

Bro-in-law fitted some to his Jeep, reversed up to another Jeep, towbars
aren't always 'seen' by the sensors, which resulted in a cracked bumper.

He then decided to learn how close he could get to a garage door from just
the indicator on the dash.  He eventually could get to within 2 inches
reliably and repeatedly and now swears by them, not at them.  ;)