I don't see it at all. He moves pretty well. Just because someone is tall doesn't mean they move poorly, have poor endurance, and can't handle a low bounce.
I agree with Twisted. He's "only" 6'6" and I think he'll be more Safin than Raonic or Isner. He's a string-bean right now, at 19, but he'll fill out and get stronger.
^ Agree with @Moxie... but I see where you're coming from. A lot of the taller players seem more injury-prone. We'll see with Sascha. Hopefully he can grab some majors in the earlier part of his career.