I've built ukes before using all Hawaii grown woods. They're fun. Ulu for the sound board, Milo for the sides and backs, hala (male pandanus tree) fret board, koa neck, kiawe bindings, etc. The problem for me is two fold. Availability of these woods can be sketchy as they are not grown or milled on any large commercial scale to the extent that koa is for instance. Most of what's available to me is the result of downfalls on the road or in someone's yard. There use to be a lot of milo more around but not so much any more. The other issue for me is from a business stand point. Since I don't work with these woods on a daily basis I don't know how these woods will respond five, ten or twenty years down the road. It's best to stay with woods that I know well. Unfortunately that means I can't afford to experiment too much.
BTW, ulu (bread fruit) is a killer tone wood. I also think monkeypod is under rated.
Ooops, southcoast responded while I was typing. Good call on that monkeypod. Jeffery Yong has received high accolades for his all monkeypod guitars, having won a blind sound test in 2006 at a GAL convention. Still, I don't have the experience to know how it would be as a neck wood.