In my opinion, sensors were a weak link with ecobee. I installed my ecobee thermostats in December of 2015, and I started experiencing sensor disconnects immediately. Since then, I've had four out of six sensors replaced by ecobee. One of the sensors they replaced had to be replaced twice. However, since last summer, all sensors have been working fine. I just had two sensors disconnect the other day, but the battery replacement fixed that issue.
Hopefully, ecobee finally fixed the poor sensor quality issue that they had, and the sensors that they are shipping now are not defective.
The ingenuity of the ecobee thermostat is in remote sensors with occupancy detection. I don't know of any other smart thermostat that has remote sensors. Without remote sensors, ecobee is just another smart thermostat - nothing special.
Most of HomeKit issues (besides disconnecting from HomeKit) are the issues with HomeKit rather than with ecobee. The advanced features that ecobee has are not supported in HomeKit. In my opinion, HomeKit is a half-baked solution that is more of a toy than a smart home control center. The Apple Home app is a joke compared to the third-party Home app. Unfortunately, Apple still hasn't opened the smart kit geofencing API to third parties. Seems like HomeKit will follow the fate of the iTunes app and iCloud photos. Great idea but poorly implemented and never fixed.