"Peter Dohm" wrote

I think that the point of "(well mostly)" may have been that even the
tried
and true combinations, such as certain Lycoming engines with approved
constant speed props, failures can occur long after the design appears to
be
mature, and some of the failures may yet prove to be related to resonance.
The moral may be that everything is experimental, but some things are less
experimental than others.

It was noted (I think it was here) that when engine mount bushings wear out,
the properties of the mount/engine/airframe combination may have changed
enough that the resonance may become a factor again, where it was not - when
the mount bushings were fresh.
--
Jim in NC