At 27, the rider should still be pretty resilient. Smaller bikes take more effort, concentration, and alertness for long distances and fatigue is a factor for safety. The machine doesn't much care. If in good condition, it will far outlast you. Crosswinds blow it sideways and you have to keep it in your lane. It's smaller, so other drivers are even LESS likely to see you, so you have to constantly dodge them. Even noise is a factor for fatigue. And a fatigued driver is more prone to lapses in attention and judgment.
A young rider should be able to cope. I did. I drove all over California state with a 305 Honda. Never flinched. That would have been miserable, even painful at age 40+.
I found the CB400F rather skittish, which is why I sold it rather quickly after restoring it. Some like the "flickability", and that has it's place. Long distance driving is certainly easier with a bike that doesn't change lanes in half a blink of an eye, like if you happen to zone out for an instant to relax a bit. Anyone who has driven long distances knows that the boredom can decrease your desire to "stay on top" of a skittish bike. Bigger bikes allow one to approach a relaxing state (for a time) while riding, which leaves you with some energy to do something at your destination, rather than find a bed and sleep off the constant tension; recharge yourself.
That's my view, anyway.
Cheers,
