I would think that even if everything is rigidly coupled the vibration will be changed and possibly delayed in time depending on what it is traveling through.
The designer of the Onedorf turntable (which uses a separate armpod) said the same thing when I rehearsed for him this (Lewm's) argument in favor of rigid coupling. Basically, as you say, the wildly differing shapes and materials of the tt motor and tonearm will prevent 'vibrating in harmony'.