I guess it depends on how you look at it.
All bikes can be customized to some extent, but Harleys are almost (no insult intended) like lego sets. If you don't like what you have, you can easily take the whole thing apart and make it into something completely different that you do, and something that's truly custom to you.
For example, imagine a Harley with Race Tech Forks, Ohlins shocks, sticky sport tires, high performance disk brakes, and electronic fuel injection--it would be a completely different bike relative to what it was.
As for motors, in all fairness, the Evolution motor ("EVO")
which came out in the mid 80s was a complete redesign and a big jump from the old shovelheads they replaced. The Revolution motor (in Harley's VSRC model among others)
was designed by Porsche in Stuttgart(!)
I'm not saying I'd give up my K13 for one, but I am saying I can understand (at least in part) the appeal.