First, I want to say that I'm answering a question, I'm sure the bio wheels work great.. I know people who will use nothing else, I've seen their tanks and they look great! But to answer the question as to why I wasn't sold... it's been my Personal experiences?
I've very displeased with my Eclipse 12. It took forever to Cycle compared to other tanks I've done using Aquaclear and Canisters. Then it finally DID cycle and I replaced the filter pad, I went through another mini cycle which took another 2 weeks. Apparently most of the bacteria were on this pad, which left me to question the effectiveness of the bio wheel.
(I also find the eclipse a pain to work withm though the acrylic does look nice). Even now, the wheel is sometimes noisy, sometimes stops. I want to get rid of this tank. I will say this... I've never had cloudy water.
My experiement with the Emperor proved to be noisy(defective impeller?). I replaced it and found that the water return as much noisier than the Aquaclear... even with the water all the way up. This was true with the wheel installed AND removed. I also found it quite bulky, and in appearance the media configuration looks weak on Mechanical filtration(though I haven't seen cloudy water). The wheel is another moving part, if it stops and dries out, you may have a mini cycle if you don't catch it in time. The emperor has spray bars to help out, but I see this as another item to get clogged/be cleaned.
I don't know... after my experience with the eclipse, and my first encounter with the emperor, I just exchanged it for an Aquaclear. I'm sure it would work just find though. People who I strongly value their opinions swear they are the best HOB filters available. But I'm not sold that the biological filter is any better, if as good as 2 sponges in an aquaclear.