There are issues with the variable UV protection offered by the different types of knitted shade cloth and whether these factors meet the somewhat arbitary "94%" factor selected by the Cancer Council. There are many issues relating to meaningful testing of fabrics (stressed or unstressed, angle to the sun etc). There are also compromises involved such as the insertion of strips to increase the blockout but which doesn't add to the mechanical strength of the cloth.
There are issues also related to the prediction of wind loads and hence the structural design of adequate support systems.
Also of concern are the uneducated expectations of clients such as schools and the need for regular inspection and maintenance.