There's nothing inherently wrong with going to a one-piece driveshaft. BUT both
bigjimzilll and
Longhorn Man are on the right track. Too long can be a problem, but not simply because of the length. It's in the balance (more difficult to achieve properly on a looong driveshaft), the natural frequency of the driveshaft and a few other factors.
Go to this link and read a little:
http://www.wallaceracing.com/driveshaftspeed.htm
One of the biggest factors is the driveshaft's critical speed. That's what makes the driveshaft live or die in a 'hot' vehicle. Exceed the driveshaft's critical speed and things can go boom real quick.
There's a story in a recent issue of one of the car mags I subscribe to (Sorry. Too many to remember which one). A guy had built a hot engine and done some other mods to his truck. He had it on a chassis dyno doing some final tuning. He's spinning the engine up (and wheel speed - chassis dyno, don't ya know) and all of a sudden BOOM. The driveshaft came out. Ruined the transmission and a some other pretty serious damage. Could have hurt somebody. Lucky! Why did it let go? He was spinning the driveshaft up beyond 120+ mph. It was a stock driveshaft designed with a max speed of less than 110mph in mind. His combo and mods had enabled the truck to far exceed what the stock combo would do, i.e., exceed the driveshaft critical speed. Moral: for 'hot' cars/trucks, get a driveshaft designed for the intended useage.