I bought a home that had an Ecobee already and was super excited because both my brother and his ex-wife have them (including a vacation home) and raved about great they are. Well mine must have been a lemon because it was total junk. The temp was off anywhere between 3 and 10 degrees, and yes I had this confirmed by an HVAC tech. The unit has an offset compensation setting to correct but that does no good if the unit itself isn't consistent. Sometimes it would read 8 degrees too hot then an hour later show 4 degrees too cold... it drove us insane.
In addition, for reasons I could never figure out, despite turning off all the "smart" features it would come on even when the temp was in our comfort range. For example we would set it to 75 but the A/C would suddenly come on at only 73. Seriously WTF is up with that?
Sure being able to see and adjust the temp on my iPhone was great but when the unit thinks the temp is 8 degrees different then reality why bother? We nick-named it the Eco-liar. After three weeks of this nonsense I ripped the thing off the wall. A thermostat only needs to do TWO things: know the current temp and switch to A/C or heat as needed. My Ecobee FAILED miserable at those simple tasks. I replaced it with a "dumb" Honeywell unit and all these problems went away so I know it was the Ecobee and not our HVAC system.