Well not so much negative as risky. If not setup and maintained properly DSB
can become hazardous. Known to be a nutrient sink and toxic gas trap. You don't want to disturb a DSB
because you can release these trapped gasses/nutrients that can kill your livestock. At the same time you need proper cuc to sift the sand. In the event of power failure a DSB
will continue to consume Oxygen to really low levels which can kill fish as well.
If you really want a DSB
then be sure to research proper setup and maintenance. I just don't think the extra nitrate to nitrogen benefit is worth the risks. There are other far less risky methods of removing nitrates. First being a macro algae fuge.