What do you mean by "efficient"? That word is usually used to describe the fraction of energy that goes into a machine and is then not lost to heat. Since the purpose of a heater is to produce heat, it's pretty much impossible for it to waste energy.

Some things produce more heat than others.

For passive heating, anything puts out exactly as much heat as it consumes electricity; there's no way around thermodynamics to gain or lose power there.

Yes, there can be heat pump systems with e.g. underground connections, and pumping heat can be more effective than generating heat; but for pure heating, generating heat, the simplest resistor heater is just as 100% efficient as any complex electronic device, all the electricity spent results in just as much heat.

And you know what happens to that energy when it's done doing its useful work? It's dissipated as heat :)

It's exactly as efficient as heating is with a resistor. That energy spent switching transistors is spent as heat. :)

Some energy gets turned to light inside ba MOSFET but that quickly turns into heat too.