One of the best attributes of coir in Horticultural applications, is that it is totally bio degradable, and can be disposed after use with absolutely no adverse impact on the environment. Rockwool on the other hand is very hard to dispose of. If it is buried, it will last indefinitely.
Rockwool can also cause PH shifts in the surrounding soil which can in turn impact the long term yield of the crops. Rockwool has a high pH which means you have to adjust your nutrient solution low so that the root zone is neutral. Rockwool is also susceptible to pH shifts requiring more routine maintenance to keep the pH levels correct.