Preheaters which involve a lot of hot air work well, as long as they are properly used. If the hot air involves actual fire, it is obviously very undesireable in the engine compartment. I have ferried for repairs two aircraft which had wiring fires in the engine compartment, bacause of the over zealous use of "propane" heaters.
Unfortunately, one of the three areas of the engine, the crankshaft area, is also the deepest in the engine. Cylinders and oil sumps are relatively easy to preheat by various means, but it takes a long time for heat to warm the crankshaft.
I use the proper electric oil sump heater, and switch in on for at least three hours before I fly. The heat rises well from the oil sump up, and throughout the cowl, and warms the whole engine. Those who blast in a whole bunch of hot air (least effectively into the upper cylinder area) for a "short" duration (less than 30 minutes) are probably fooling themselves. That heat has had little effect on the crankshaft, which is the most expensive part of the engine to be turning over with ineffective lubrication.
Slow and easy does it best.... like so many things in aviation....