Lots of tricky engineering problems, but it's clearly been done both well and badly. Lots of subtle issues with tree growth, movement in storms, etc. One that's been built on the next island over from the one we're on can be seen here:
Oh man, did we have fun building tree forts as a kid.
The local news had an article on a tree house that was challenged by the local gov't. I think that he removed it when the fines began. It wasn't a living space.