the bio wheel is fine for nano systems. it makes the most sense when you dont want to have a sump on the real small set ups.
anything can be a nitrate trap if you dont take the time to clean out the gunk once in a while when it builds up. i have a larger biowheel model on my 29g QT tank, used in conjunction with a maxijet 600, the GPH is fine and the trates are nill to manageable, depending on the bioload.