Well, you will have to stop to refuel unless you can find someone to refuel you while driving.

Sleep can be an issue too, but I know some people who switch drivers while driving, so you may catch some sleep while someone else is driving.

Food and personal hygiene could be an issue too.

Joking aside, I can't see why it would be a problem. You still stop and accelerate so you get to work all the gears. Most people I know drive the same speed all the time, typically 10 over the posted speed, only slow down for traffic and only go over to pass others on a two lane highway.

I've never heard of anybody having problems because they always drive the same speed. I also never heard of anybody driving different speeds to avoid damage.