Many yrs ago a dealer told me never to use those ultra-sonic brushes for they could vibrate the cantilever and possibly damage the suspension. I threw mine out many yrs back.
Vinyl grooves vibrate the cantilever...up to the suspension...also up into ultrasonic range. I guess the question is whether relatively high energy levels at ultrasonic frequencies are more damaging than even higher energy at low frequencies (which suspensions are quite normally subjected to). But it sounds like the dealer was simply speculating, as we all are a this time.
I reject the notion that seemingly "harsh" cleaning methods are dangerous (when applied properly). The stylus and cantilever have to be able to absorb relatively large forces in normal use. Cue up into the outer lead groove of a 45 RPM 12" and watch it "slam" into the 1st music groove. Any stylus that’s knocked out by a gel dip or a US cleaner or a vigorous brushing (etc) was on its way out very soon, anyways.
When Consuela rips off your Dynavector XV-1s stylus with her feather duster, that’s about 100 - 1,000x more force than the forces described above. These assemblies are delicate on that scale. They’re not so delicate on the former scale, except by manufacturing defect.
I love that my old dealer showed me how he brushed the stylus clean, all the way up the cantilever to the front plate. It was a lot less light-handed than I would've guessed. I would've been too skittish to properly clean if I hadn't seen that.