I do a lot of decks, mostly composite, almost always Trex & some TimberTech. I really like the KSS 400 for squaring up the ends of the material before installing. It looks like (from your pic) that we install the same way- picture frame the perimiter with miters (most here run the skirt all the way up even with the top of the deck boards), run a piece perpindicular at a good breaking point if the deck is larger than 20' wide, hidden fasters etc. My typical job & the tools I use are as follows:
Frame- Wormdrive circular saw- Skil LT
Perimeter- Skirting and Mitered picture frame- Usually use a sliding compound miter saw with a sturdy table. I use the KSS400 sometimes but find it to be easier and more accurate to hit the exact line while mitering with the slider and stand.
Decking- True one side always with the KSS 400 right off the stack, butt the perimeter and run them wild then cut true with the KSS 400 and long tracks
Planter- Any items like the planter boxes in your pic I set the stops on the miter saw for the gray verticals and also cut the 45s & 22s on the miter saw- faster & more accurate
Stairs- always the miter saw with the stops set.
Possible issues- The KSS 400 will clog easily with the Afro dust from the Trex if you cut too many without the vac attached. I prefer a cyclone attached to my Festool vac becuse as you know the Afro sawdust fills up the vac really fast.
Sorry about the long reply. The KSS 400 definitely has it's place in my system, nothing you can't use the worm drive while holding a speed square for though, just a bit more convenient. Your work looks great.