[CarlC]

Yes mileage, but packaging and weight also come into play. No intermediate shaft, rag/universal joints, firewall penetrations, etc. make for easier packaging. It all affect weight, which equals mileage.

We make these at my company. IMO, they are not even close to being in the correct performance envelope vs. working space vs. cost for the cars we work on. Cost goes down on mass production scales, but it's very expensive, and legally muddy, to start dealing with these items on a small aftermarket scale.