The product is definitely unique in the way it functions, in the issues that have to be dealt with in terms of safety, in the speed with which it gets up to temperature and a host of other things. This is why testing would need to be somewhat different from conventional units. Just like when testing hybrid cars that have the capability to drive on electricity alone or switch off their engines at traffic lights.
But this doesn't mean that it is in any way different in terms of efficiency. When it comes to that it does not matter at all what they technology is. You have an amount of electricity or gas or coal or wood or whatever going in and an amount of energy coming out in the form of hot water. Knowing how much energy is stored in any of those fuel/energy types u can easily calculate overall efficiency and compare totally different technologies against each other in a meaningful way. Or do it in a more practical way and just factor in money in vs hot water out.
Same as plug in hybrid cars have a certain amount of electricity going in, a certain amount of gas going in (in some cases) and a number of miles coming out. You can very easily compare the cost to run for a year with the cost for a traditional car with no fancy tech. And just like in the case of the water heater plug in cars work best in specific scenarios and no so great in others so different use cases will skew the comparison towards one technology or another. And at the end of the day it also means that even if a new technology isn't better in every way than existing ones it may still find a place in the market and thrive. So if this heater is not significantly more efficient than existing tankless electric ones when run on a traditional always-on type test (which i don't believe it will be) it will not in any way mean it's a failure.
As far as it heating water efficiently enough to be marketed as a water heater... Really depends on your definition of "efficient enough". Personally I think there was never any doubt that it can heat water and it can be called a water heater. The question is exactly how efficient it is (specifically is it competitive), how much it will cost, which scenarios is it best suited to, what are it's advantages, etc etc.
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