alt Hacker NewsEarly generation Nest hardware was garbage, and was known for blowing FETs that failed closed, turning people's ACs into giant ice cubes. Putting it in the landfill would be doing yourself a favor.
The ex-Apple culture in the early history of Nest was evident, which ostensibly spec'd FETs over mechanical relays for superficial reasons, because clicking sounds are ugly. The results were in the spirit of other Apple engineering marvels (Titanium Powerbook, Antennagate, Bendgate).
Well that's certainly a take. Solid state relays using optoisolated MOSFETs have been around for fifty years. Mechanical relays are overkill for signal switching as in HVAC thermostats, IMHO, but you do you.
Anecdotally, I have a first generation Nest and haven't had a problem. Maybe some of the earlier hardware had fewer protection against misuse (e.g., with non-24VAC systems or otherwise incorrect installation), but that's generally the case with most new things.