Given that the Falcon was haphazardly modified by Han, who is the only person in the galaxy (until Rey) who understands how it's put together, don't be too harsh on poor Chewie. He's doing the best anyone can with that hunk of (fast) junk. Even Han couldn't explain why things weren't working at times, and it's his ship and his mods, so Chewie beating on the ship doesn't mean anything.
As for Chewie putting Threepio's head on backwards, there's two possible explanations other than incompetence. One is that after having the head torn off violently, Chewie put it back on the only way it could be made to work, using the tools that he had at the time. The other explanation is that he did it deliberately. Threepio is annoying.
Edited by ShadoWarrior