I've run quick detach setups for years on Harleys without rattles, etc... I can't imagine the R3T being any different. In fact, I can tell you the Triumph piece is a better design. The locking mechanism (the brackets, not the keylock) is all metal. HD's locking clips are hard plastic.
Also, the HD brackets slide on from the back, meaning the c shaped hooks that lock into the docking station face toward the front. That's always been a head scratcher for me because the stress (weight of passenger) comes from the front (pushing back on the rest). I've never seen or heard of it happening, but in theory if the mechanism broke, the backrest would fly off the back possibly taking the passenger with it. And, given that the locking mechanism is plastic, well...
The Triumph backrest hooks from the front like one would expect so even if the mechanism broke, it still be darn near impossible for the backrest to come off.
Finally, Triumph does make a luggage rack for the backrest.