Tom,

The only small downside is that it might be hotter and more humid. I've never been in the fall, so can't compare, but I've been twice near the end of May, and once in early April, and the April trip was definitely hotter and muggier. I would imagine March would be even more so.

This doesn't mean much when you're on the ship, of course, except that you might find breakfast or lunch at the Grill too hot.

I believe it's supposed to be the rainy season as well, but that's a crapshoot in FP, you never can tell. It's also the time when RSSC sometimes has its "summer in Tahiti" specials, so you might get a better deal as well.