I wonder if thats why i have seen this phenomenon with harleys a number of times. I'll be on the freeway and see a HD cruising along at normal speeds and it will be in a perpetual speed wobble. i have seen this many times and it's scary. Not like a speed wobble thats necessarily going to mean imminent danger, but you can easily see it. I once saw a group of 4 HD's and 3 were doing that. I once saw a HD down on the freeway with apparently no car involved and wondered if that had anything to do with it. I also have a friend with a dyna model and he isn't afraid to tell me his bike scares him in the way it handles even in a straight line on the freeway. I recently went camping with him and on the freeway he was under the speed limit by 5 MPH most of the way. I had to keep slowing down and waiting for him. he later told me he had to due to the road conditions and the fact his bike couldn't handle it well. The road was normal ! So maybe thats the issue.
I've never noticed a Harley wobble, but that might explain why you hardly ever see one doing more than 60.