As mm2 alluded to above, there's a completely different kind of vibes off any Inline 4 compared to most twins, especially V twin cruisers. Different frequency, mostly. Some can handle one but not the other. I've been on big Japanese I-4s since the early 70s, before which I was riding the king of all vibrators -- a '69 BSA 650 Thunderbolt.
Even before the days of counterbalancers, for ME, an I-4 was the smoothest thing out there, but I'm apparently not especially sensitive to their particular frequency. By contrast, that Beezer used to put my hands and forearms to sleep, and I was young then.
Some I-4s are smoother than others. I have zero problem with the FJR and it doesn't seem buzzy at all to me, or at least -- mine doesn't. Most V-twin cruisers would drive me nuts to ride for any appreciable length of time. No question, both my Blackbirds (still have second one) were smooth as glass, with noticeably less vibration than the FJR. A friend has promised me a ride on his '09 Concourse 14 this season, and I understand from him that it is much like the Blackbird (he also has one).
So, it's a difficult judgment call without riding an FJR, I guess -- depending upon how sensitive you are to I-4 frequency vibes.