Biggest draw back of oil pan heaters was coked oil and, being on the bottom of the oil pan, the tendency to get ripped off. Ice chunks off trucks, snow curbs, regular curbs hidden in plow snow, etc.
Second, if you heated the oil, then pump it through a cold block, it cools back down pretty darn fast. Conversely, if you heated the oil enough for the block to be that warm then the oil is getting pretty darn hot and burnt.
I think warming the oil enough to not be thick goo (good on older dino oils), is a good thing, but not as necessary with modern synthetics. It's already going to flow pretty well unless you are out in -50, -60F temps.
I got a FrostHeater when I live in Montana - even on those -40F mornings there was warmth right away. Nice side effect was the thermo cycling of air inside the car and hot coolant in heater core cause the windshield to partially defrost before I woke up. Just let it idle for 30 seconds to stabilize the temperatures between oil, coolant, and different parts of the engine before driving off.
I think ideal setup would be both oil pan heater and coolant heater working together, but would need to find a thermostatic oil pan heater or put it on a different timer.
Jason