I have done crushed coral before and hated it. It was certainly a detritus trap even when keeping flow moving to it....there were just too many places in a large system that became "dead spots".

Kind of low level...but one thing I didn't like was the ability to separate certain corals....with the crushed coral they just grow over it. So the privelage of putting aggressive palys on an island or even GSP became a problem. Of course that problem also exists with bare bottom.

I have also done 50/50 mix and was not happy with that either because the two mixes do not stay mixed and eventually separate making it obvious that you are using two substrates.