Actually, outdoors, one thing in your benefit is there are no room modes.  And if your setup is far enough away from buildings, then there are no early reflections.  That's a significant advantage, acoustically.  Of course, you'll probably need more power because space is not constrained.  But that's not too difficult to provide.

Seems to me the biggest issues are the elements to deal with, like the weather, insects, etc.  I assume the goal would be limited to night viewing.  Might have noise problems, either from outside interfering with your viewers or from your system disturbing neighbors or violating local restrictions in some way.  So I think the biggest issues aren't technical audio or video matters, but rather they are problems of regulation, interference, convenience and level of comfort.