Years ago there were discussions on ECF about frequency and waveforms in electronic ecig mods. Frequency turned out to be most important for a smooth vape, IMO. The higher the frequency of the synthesized voltage the smoother the vape and you could tell which ones used higher frequencies to approximate a DC voltage. There were square wave, triangle wave, and various other designs in the race for the perfect vape with variable voltage.
That BTW, was the reason that mech mods were king at one time. Mech mods utilize direct DC voltage straight from the battery to the coil through a fire switch. DC is king in terms of vape quality. Mod makers used various waveforms and voltage synthesizer chips to achieve a mech-like vape that allowed variable voltage, wattage, and eventually temperature control.
As the technology advanced chips were designed especially for vaping and could be programmed in ROM to deliver most any waveform. After that, it was more about cost, patents, features, and pricing.