there is only one problem a DSB
does wrong and thats if you disturb it while its still i nthe process of establishing itself. if organic matter gets under the sand before the life has built itself, the organics will decompose and creat toxic gases that will poison and whip out your entire system.
if done properly a DSB
mixed with a refugium and live rock will be the best filtration system you could use in a reef, as it will be the most natural. PWC
to remove nitrates and organics will not be needed unless to reestablish used up trace elements