I’ve mounted my iPhone on the handlebars of several dirt bikes (pretty much the most vibrating bike possible)
1000s of miles with a vibration dampener, and no camera issues!
I do have a ruined battery though. The positioning + wind result in moderately extreme temperatures (very hot in summer and cold in winter), so my phone no longer holds a charge for very long
Have you just mounted them, or mounted them and used the camera while riding the bike? The iPhone has sensor-shift image stabilization (what camera manufacturers are calling IBIS - In Body Image Stabilization) and if this is engaged (for example while filming) it would be much more prone to damage than if you are using the phone for, say, directions only with the system parked in transport mode.
At least this would be my assumption, I don't know where this problem comes from exactly.
V twins are way better balanced than single cylinder engines. Singles have extreme primary imbalance. V-twins are a bit primary a bit secondary but neither is extreme. Dirt bikes don't have rubber motor mounts. No motorcycle I'm aware of does.
In terms of imbalance and vibration, IMO, thumpers are the worst, then parallel twins with 0-degree cranks, then other parallel twins. Triples and v-twins are somewhere in the middle. Inline four is decent, inline 6 is even better, boxer is best.
Other bikes have isolators that aren't easy to see at first glance, as it looks like a bolt passing through the frame/engine. You have to remove the bolt to see it's floating on rubber or nylon isolators.
1000s of miles with a vibration dampener, and no camera issues!
I do have a ruined battery though. The positioning + wind result in moderately extreme temperatures (very hot in summer and cold in winter), so my phone no longer holds a charge for very long