Well, now, wait a minute. You can get the same amount of hot water with less energy, if you use a tankless heater. “Efficiency” in the context of power generally means power out divided by power in, and that’s not this, exactly, but less energy to get your hot water is still a good thing.
There are two reasons this is possible. One, the same thickness of insulation will let more energy leak out of a large tank than out of the small heating zone in a tankless heater, as the energy flowing through insulation is proportional to its area. And, two, in a tank type hot water heater, you can’t put the heater right under the tap, so to get hot water you must run the tap for long enough to clear the recooled water out of that run of pipe. And you spent energy to heat that water, even though it cooled off again since you did it.
I don’t see why microwaves would be a better way to make tankless heaters. Maybe they are, for some reason. Though, resistance heating works awfully well - I do it all the time with air. Of course, if you build a tankless heater in such a way that it doesn’t fit right under the tap, it loses one of its advantages. And any one particular manufacturer could do all sorts of slimy things.
I don’t know if you get more hot water for the same energy with the cited heater, if it’s even been manufactured. But in principle it should be possible to do so generally with tankless heaters.