I'm done with it. Whatever seems like too many joists ya better add more. Mine sagged in the heat, and faded fast for me. I can't say wood is better, but more predictable and stayed more straight by comparison.
I've got Trex on my boat dock. It has certainly held up better than my neighbors wood plank dock (built by the same guy FWIW). Neighbor never bothered to treat or stain his though. One thing I will say is that in the Florida sun is that shit gets HOT! Like you can't walk on it barefoot hot. It's got a 20 year warranty on it from fading. Also, it doesn't float, so I don't have to worry about it trying to pull the dock out of the ground if the water to ever get real, real high. Not cheap either.