Several carfentanil labs have been discovered in Canada.
The reason is someone published a synthesis that makes it's production no more complex than that of fentanyl so it makes perverse sense to go for the stronger option. It also avoids a couple of watched precursors.
Not much is known about the action of carfentanil in man but it's widely suggested to be more lipophilic and have a larger volume of distribution then fentanyl which is why the few reports I've read state it's DOA is longer than fentanyl.
It's also achiral. People love to point to the potency of analogues like OHMEfentanyl but that has eight enantiomers and while all appear to be active, it's only one of the eight that has the potency often ascribed to the compound.
@axe battler - I admit to having dropped a pill on the floor only to find it months later and the funny thing is that oxycodone seems more active orally than via a parenteral ROAs based on an experiment. That may be because although OC is active in it's own right, first-pass metabolism will convert a fraction of it to the much more potent oxymorphone. But overall, it does appear to have addictive potential out of all proportion to it's analgesic activity. I just take as prescribed. I have the strong impression that if abused, it's a faster route to addiction than even H. Only Palfium struck me as a worse 'trap'.