My Beckman/ Needleworks.. stays solid as a rock because their mounting is a 3/4" webbing and buckles ..
Strap is sewn in the end to form a loop, to pull the strap through the buckle, with your finger,
and there is velcro sewn on the side of the loop to stick the loop to the back of the pannier,
to keep the straps out of the spokes..

Yea Ortlieb has the simpler to use mousetrap.. the Beckman setup is secure on the bike
because its fitting scheme is not easy to remove, for the unfamiliar 90%+ of the population.

My one-off rain covers also had the PIA security system..