Biowheels being wet/dry means they can only support aerobic bateria colonies .... sandbards (both shallow and deep) will house aerobic and anaerobic (oxygen deprived) bacteria. The balance could be upset from the natural chain of events as the bio-wheel will convert ammo --> nitrite --> Nitrate and dump it into the tank as there is no anaerobic areas to consume nitrates.

Maybe look at it this way, if the biowheel consumes most of the ammo and N02 from the tank, there may not be sufficient nutrients to feed the LR/SB bacteria, throws the whole balance out of whack.