[gnuplustoejam]

Early 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).

[mechanicalpulse]

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.

[gnuplustoejam]

Sounds like something Nest engineers would have said.

It's not "signal switching", you see.

HVAC equipment is as old and varied as you can imagine, and there is higher current than you think running through those terminals, powering all sorts of nasties, oil burner relays, damper motors, crude AC contactors causing voltage spikes etc. HVAC low voltage power is as dirty as can be.

No one took this into account, they were more concerned with making the thermostat pretty.