There should be no problem with using an ecobee. Many folks here use smart thermostats (myself included, I use a NEST).
Do you have a second stage/backup heat source?
All the ecobee products I have looked at have standard 2 heat/2 cool capability.
Thanks.
Excuse my ignorance but what do you mean by having a second stage/backup heat source?
Right now I have four sets of thermostat as follows:
2 thermostat control each of the Heat Pumps for normal operation (that is NO OWB involved)
2 separate thermostat control the temperature at which the fan blower comes on/off from the two above mentioned heat pumps during OWB winter operation when thermostat calls for heat.
Many thermostats, most every smart or programmable now-a-days has the ability to control two or more heat or cooling sources. It's basically two thermostats built into one. Some have interactions between them so you can determine when/how each heating or cooling source is activated. It saves having to manually control multiple thermostats.
Sounds like what you really need is a 3 heat / 2 cool thermostat. You have two stages of heat pump heating, and one stage of OWB heating. Likewise you have two stages of heat pump cooling.
You could go with the Ecobee3 Lite:
https://www.amazon.com/Ecobee3-Thermostat-Wi-Fi-Works-Amazon/dp/B01K48T09Y/ref=pd_sim_60_3?_encoding=UTF8&pd_rd_i=B01K48T09Y&pd_rd_r=XTJFR0K99ATNHTKXH86Z&pd_rd_w=Ty01P&pd_rd_wg=BubAz&psc=1&refRID=XTJFR0K99ATNHTKXH86ZHere is a connection example for radiant, and 1 stage heat pump heating W2 would go to heat pump W1, not W2 on boiler, and 2 stages of heat pump cooling:
https://www.ecobee.com/wp-content/uploads/2016/10/ecobee3lite_wiringdiagram_boilerradiant.pdfThe main problem is that heat pumps are treated by thermostats to be the primary source (stage 1 and stage 2 of a 3 stage system). I don't see any Ecobee products that have 3 independent heating stages. W1, W2, W3.
NEST does have a W3 option, as do some other 3h capable thermostats. Are you fixed on Ecobee or would any smart or programmable thermostat do? Do you want a separate app to control, Alexa integration, ect..?