I don't see why one might consider them inferior to loose bearing BBs. I have one (Stronglight) on my bike and I have never had any problems with it. Most of the ones I encountered have very very smooth turning action and if the bearings ever wear out, you can either just replace the whole unit or press on new cartridges. Some have noted that sealed bearings have drag because of their seals. but it is so slight that I don't think any riding mortal can detect it, specially on a relatively low RPM component like a BB.

Chombi