This is simply a blindingly good business plan, exotically gorgeous in fact. For of course the cost of the electricity required to mine for bitcoin is very much larger than that 25 per cent of the value of the bitcoin that we get to keep.
For anyone who has to pay for their electricity, this is a horrid idea. If the heat generated from the bitcoin miner isn't directly beneficial to the user, then the cost of the electricity spent mining must be weighed against the bitcoins earned. With a decently efficient chip, and with 100% of the results kept, this is very nearly a wash right now. At 25%, it amounts to theft.
This type of thing could make sense in a space heater or coffee warmer, but that's about it.
You bring up a fair concern. For some people the cost of electricity does not justify the savings on the free appliance and 25% mining /tx fees rewards. I would expect that there will be reviews and guides which go over the numbers and indicate which appliances are most efficient and agree the items which are used for heating like space heaters , coffee warmers , dehydrators, water heaters , ect... are likely to become the best candidates.
With 116million in backing and an aggressive campaign to make many different types of products they have a good chance of finding at least one product that is a home run and gets adopted by many.