I bought a recently publish Asset (Bezier Tubes) and discovered that the documentation was almost non-existent. I've found no way to use the new Asset without producing errors.
The publisher website has a non-functioning Contact button, so I have no help there.
Directly complaining to Unity is of little help, you tend to make users "work it out" with the Asset publisher rather than just give a swift refund on crappy assets.
My Suggestions are:
1) Plus and Pro users should be given a "No Questions Asked" Refund Policy on purchased assets. It would encourage me to buy more assets knowing that I could simply "undo" my purchase when an Asset didn't meet my expectations. As it is, I am extremely hesitant to spend more that a couple dollars on individual assets.
2) Tighten your Quality Control on assets listed. Can *your* team members make it work? Does it pass a basic set of quality standards. Is the documentation complete? Is there an easy way for the customer to contact the Asset publisher?