I would also suggest searching "trex" in this bulletin board. You will find several posts about a member who had a serious issue with a client.

There can be problems with a lot of different composites. The earlier and cheaper generations of this stuff is known for problems. Basically most of it has wood pulp mixed in with the plastic, and the wood fibers get full of algae. When we clean it the fibers bleach out and you get a lighter color than many customers expect, like white flecks or "fuzz" in the boards. Also, if it has ever been pressure washed, you are likely going to uncover damage, raising or fuzzing of the wood fibers, or etching of the plastic, or probably both. Like most home exterior components, pressure washing voids the warranty and is not recommended by the manufacturer.

That all being said, SoftWashing is the best, and really the only way to clean this stuff. Just be extra sure to really lay it all out to the customer. It might even be worth having them "sign off" on these possible issues. I run across these deck at least weekly, and they are a pain, and take a lot of "coaching" of the customer's expectations.