UK has had the basic "fit for purpose" since the 1979 Sale of Goods Act.

At least in the UK there are the concepts of "fit for purpose" (and of merchantable quality) and "reasonable expected life". Now I doubt there's yet been much chance to evolve what a reasonable life of a smart thermostat or associated devices are, but crucially liability is with the retailer. It's on them to prove they are not liable. If they then want to chase the manufacturer, that's separate. It probably pays not to buy direct from manufacturer.

A retailer can be busy ducking all their legal obligations and telling you that you're long past the window of refund, but mention the provisions of the Sale of Goods Act and you usually get a very different response, or a manager is called over (to authorise the inevitable refund). The Sale of Goods Act is still law (Well, parts of it), and the newer EU Consumer Rights Act is in force as well. Good job too, the Sale of Goods Act is stronger in several areas, whilst the EU gave us 2 year warranty.

There is case law where things have been judged to be within reasonable expected life, and a repair, refund or compensation ordered long after the mandatory warranty ran out.