Surely crushed coral is beneficial in some ways!! It looks good mixed with sand in posts I've seen
Let me explain. Crushed coral is made from coral that has been smashed up in to tiny pieces. Those pieces are very sharp and totally detrimental to any sandbed organism, including fish that like to burrow. Also, it does tend to trap detritus within its sandbed because its coarse shapes hinder it from compacting. This results the bed's inability to form an anaerobic environment which, in turn, stunts denitrification. CC beds = Very High Nitrates. Nitrates are a waste product not desirable in marine tanks.
The goal is to establish a marine system that is free of ammonia, nitrites, and nitrates. To accomplish that it is best not to be constantly battling poor water quality because we have equipment and other items (such as CC) in our system which obstructs our efforts to establish a healthy marine environment in our tanks.
Now in some systems, a tad of nitrates is unavoidable just because of the nature of the type of tank being set up, but, with CC nitrates will soon go off the charts in a fish tank.