Each time I see one of the "big singles shake" comments I suspect that the writer has not used the ZDZ80 singles.
When these engines are setup acording to spec - and properly adjusted - you can do full run up tests with out the wings on the plane.
I do it all the time with models that are 18 lbs to 23 lbs .
Other hard core twin fans have commented on how smoothly these run.
I also suspect that many other engines (singles) will do the same setup correctly.
I have seen 100 twins which shook much more.
They also vibrated (different problem)
The old inline twins I worked with really sucked - they were the ST 6000's and were , frankly impossible for the "engine impared" modelers to make run correctly.
There are some new inline twins on the market -which from a purely theoretical standpoint ---should---be smoother.
simply because there are two power pulses per prop revolution
this means that th prop should maintain speed better -reducing the reaction that occurs as compared against a engine of same power, firing once each revolution.
( look at a large displacement four stroke single to see how this can get out of hand.)