Murray, Advantages:Transmission out-put shaft can be the same as other cars/truck & input of T-case doesn't have to match out-put of transmission, driveline (driveshafts) angles can be reduced due to mounting T-case lower, can fit under flatter floor.
Disadvantages: Extra driveshaft/U-joints to wear out or break, extra mounting brackets/crossmembers needed (all add extra weight), T-cases seem to shake/move around under power. I'm sure there are some more to add, that's all I can think of for now. Lorne