The failure is always the colour
Nobody notices a matched job. Everybody notices a deck that is slightly warmer than the screen beside it, or a soffit that reads greener than the fascia under the same light.
Powder coat varies between batches and between manufacturers. Two suppliers quoting the same colour reference will not give you the same colour.
What ordering together actually buys
One batch. Decking, fencing, roof trim and ceiling panel coated together, so the colour is the same object rather than the same code.
Profiles that meet properly. Edge trims from the same system are designed to close against each other. Trims from two systems get filled with silicone.
One delivery and one point of blame. When something is short or damaged there is one number to call.
Where it matters most
Anywhere two systems meet in the same sightline. The deck against the fence. The ceiling panel against the roof fascia. The screen against the planter edge.
On a job where the deck is inside and the screen is outside and they are never seen together, it matters far less. Say so and we will not talk you into it.