largely true for the British although remember the Vincent came back post war and the Europeans were making plenty withing 25 years or so after they recovered from the war. It's hard to make judgements like this for a number of reasons primarily because of the economic dislocation after WW2 esp in Britain. Money poured into Europe to rebuild and hedge against the Russians and Communism and Britain had a war debt to pay the US.
Having said all that the British motorcycle manufacturers were totally hopeless post war in just about every way, from production, to economic management. All the money went on Oak lined board rooms, Company Roll Royces and big salaries for the management instead of R&D and new machinery.
Remember the Japanese didn't have any problems with construction costs producing the Honda 4 in the late 60's early 70's from a clean sheet when the British were pumping out twins whose designs dated back to the 50's and before and the Americans were making Harleys still.
I've owned 3 Aprilia V twins ( all RSV1000's. 2 R's and a std RSV) and been inside them a few times including club racing bikes and they are bullet proof. If it blew up to that extent I can pretty much guarantee you it was an owner/operator issue.
As for real power a Ducati Panagale makes close to 200 HP. A de-tuned one making 150 would be pretty docile for a sports tourer if they had the inclination.
Ciao