This isn't to say one can't ride to say New York on a 500cc bike, or the world's best Wee Strom, or a Ninja 250, whatever. It can all be a challenge and fun. But if I'm going to spend two weeks riding the Western States, I much prefer the larger bikes.
And that's actually exactly what I did.. Ninja 250 from NC to CA/OR, and back again.
I wouldn't ever do it (on that bike) again...
Many folks fail to realize that there is weather outside of California. When you're touring, you'll experience weather. Nothing makes riding in weather easier then a big powerful faired machine.
Give me cruise control, heated grips, electric windshield, fairings, a comfy seat and a smooth powerful engine and I can eat up miles through all kinds of weather.
^ and THAT is the reason why.
The Ninja 250 happily went across the country, but my body started to say "No." somewhere in South Dakota, with 80mph limits and a 20mph headwind. It was absolutely miserable, and I would have paid good money to jump in the shittiest looking car that went past me.
Long trips wear you down... The big bikes and fairings/windscreens prevent that from happening. It isn't just the weather (rain etc), but the constant turbulence, headwinds, hot days, frozen days etc... it really starts to take its toll after a few weeks in the saddle.
You can tolerate a lot of bikes for a couple days... if you slow down, then even longer... but when you really want to make time, and do it consistently, then it gets really hard to beat a designated touring machine with full wind protection.
Think carefully about the kind of ride you want to do. Will you be out for weeks at a time? If not, one of the sportier bikes with less protection would probably work just fine.
If it had a bigger gas tank, I think I might really enjoy something like the big VFR1200F. They didn't sell well, but are incredibly high quality bikes. Shame about the damned tank size though.