I had Nest Gen 2 since Aug 2013. No failures at all. I did not bother with self learning mode.
5 years later in Aug 2018, I decided to replace it with Nest Gen 3 and 3 sensors to better control the temperature in my house. 2yr warranty.
Mar 29 2019 Battery stopped charging.
Mar 31 2019 Google shipped Nest replacement. I received it in 2 days and shipped the bad unit immediately. I kept getting reminders that they did not receive the return even though I had proof that it was delivered. Finally they acknowledged the receipt of return on Apr 22nd.
Aug 29 2020 11pm. I saw that Nest was offline on the App on my phone. I went and saw that it was not connecting to wifi. Battery was low. According to instructions from the previous experience of bad battery, I charged it not for 1hr but 9hrs using micro USB and at 8am next morning battery was only 3.75V which was supposed to be enough to connect to Wifi. The unit would not see any wifi networks including hot spot on my phone inches away. Apparently Nes gen 3 has a known issue of failure in wifi chip on top of draining battery.
Aug 30th Sun 4pm. I received return label for the bad unit and I was promised a replacement to be shipped in 1 business day. Warranty was expiring on that day! They took another week to let me choose how I want the replacement. 1. Return the bad unit and wait till they receive it and then they would ship a replacement. Note that it took them 3 weeks after I shipped the bad unit for them to receive and then few more days to get replacement. 2. Ask for Advance Exchange and they would send me a link to order replacement unit for $190 at put a hold on my Google Pay account to that extent. I asked for Advance exchange. They took another week to send me a link to order using Advance exchange and then another week for me to receive the replacement unit. The return is expected by Sep 23rd back at Google and I have no idea when they will actually remove the hold on my Google Pay account based on previous experience when I get too many reminders to return the bad unit if though I did immediately.
No replacement shipped even by Friday Sep 4th. We had 3 days of 110 deg weather forecast Sat to Mon. I went to Home Depot and bought Nest gen 3 and it was working perfectly. I asked Google if they can give me credit for the unit I bought. They would not.
Sep 19th I received the replacement unit from Google and warranty would be extended for 90 days.
I returned the unit I bought at Home Depot because Google asked me to "work with the retailer".
If you do not use the so called learning capability of Nest you have other choices from real thermostat manufacturer Honeywell Lyric T5+ if you do not need additional room sensors. It $70 cheaper than Nest gen 3. There is Honeywell with 1 remote sensor $200 and 2 more sensors can be added for $65 vs $220 for Nest Gen 3 and 3 pack of sensors for $100. $320.
There is also Ecobee and I know people who are very satisfied with Ecobee and Honeywell. If my Nest gen 3 dies after 90 days, it will be replaced by Honeywell.
Summary: Google Nest has reliability as well as very poor support and you have other choices now.
One final thing I did not really with any support from Google. A 24V transformer connected between C and Rh provides 200mA to keep the unit always powered rather than depend on 20 or 40mA provided if you do not have C wire connected. There are other ways to connect C wire you can find on youtube.
Also, you can find videos on youtube how to replace the battery on Nest. I ordered one. You get the idea? If you buy Nest, better be prepared to fix it yourself or just by some other brand.
