The reason it "works" is that the farmers using this had long term, fixed electricity contracts.

So they can mine bitcoin using the cheap (relatively) pre-gas crisis prices.

But yeah, in practice it's a massive waste of energy and natural gas. Because the Netherlands generates about 60% of it's electricity from gas (and another 15% from coal), with renewables, nuclear and biomass making up the rest.

So what happens is that instead of using 1 unit of gas to provide 1 unit of heat, now you use 2 units of gas to provide 1 unit of electricity which gets turned into 1 unit of heat. This strategy is increasing fossil fuel consumption, and it only works because the cost of extra consumption falls upon the power provider, not the farmer.

Capital wise probably not that efficient. Energy wise it's the same as resistive heat, all electrics are 100% efficient when it comes to heating. However if you were actually looking for energy efficiency a heat pump is the better solution because they actually get beyond 100% efficiency due to how they can take heat from outside and put it inside (even when outside is colder than inside).

Also if you aren't on a green grid direct combustion is more efficient than resistive heat because of power generation inefficiency, but heat pumps can often still come out ahead