I've only used my HKC on a small amount of azek, but I didn't run into any issues. Its not hard to hold the saw/rail in place while cutting.
As for composite decking. I'm sure you know composites are pretty hard on blades. The HKC only uses a single 18v battery so you'll end up lacking a bit on power after the blade dulls/gets dirty. On smaller projects its fine, on larger decks you'll want to clean the blade(s) and have extra batteries.
As I said the HKC only uses one 18v battery. The power isn't bad for a one battery only saw, but its not great either. There are times when I love the HKC, and times its just doesn't feel like its up for the task and I have to cut slower than I'd like. My advice, if you decide on trying one out buy local and use it for 30 days as Getmaverick said. If it doesn't work out don't hesitate to return it. Remember, only tools are returnable after being used, all accessories are not. That means the blades and extra rails are not returnable once used.
Good luck and be sure to let us know how things go if you decide to give it a shot.