The classic answer is that a high compression engine feels harsher than a low compression one. The high compression gets you more speed (and more to the point, better fuel economy) but the low compression is more forgiving and smoother. This applies to all engines, not just Square Fours. I have had a couple of periods in my life when I have run 2 bikes with essentially identical engines, apart from the compression ratios. Both times I have found the low compression version the pleasanter to ride. I should add that I am no speed merchant so the extra 5 or maybe 10mph at the top end means nothing to me but I do like a smooth bike. My RH500 is low compression and lovely to ride, so much so that I find myself resting my left hand on my leg, rather than on the end of the handlebars. I have only ever done this with one other bike - also a low compression 500. But I ran the RH500 as a high compression 350 for many years (probably over 20) and never once in that time found myself with the left hand resting on my leg.
What is also evident though is that some people are much more tolerant of harsh, vibratory machines than others. I will not name names, but I can think of two very well known names in the Ariel Club and for each of those, I have ridden two of their bikes and wondered how their owners put up with them. Yet the owners in question did many thousands of miles on these machines. To be fair, my own KH vibrates badly - I know why and how to cure it but have yet to find the time to do so. Meanwhile, I avoid using it but its sheer convenience factor means I still do almost as many miles on it as my other bikes.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.