The problem with the nest, is that it's s thermostat that controls simple systems.
Meaning that anyone who has a high efficiency air conditioner (2-stage), and then trys to install the nest, has problems.
Duel fuel, your screwed.
Using a programable thermostat on any cast boiler is something I suggest against also.
The only people who can use the nest, are people who have standard AC/gas furnace, or standard Hp/Air handler.
People who buy a Nest thermostat, are usually the ones who want high efficiency air conditioners. Those "green" types.
Meaning, alot of the Nest's market, can't even use the stat.
Our company's market is very focused into non-standard systems. Geo, high efficiency a/c, Duel fuel, water + steam boilers, Air handler/boiler/heat pump, radiant infloor, mini-splits. The majority of our forced air have dehumidification, on-demand humidification. In 7 years, I've installed 2, 80% gas furnaces.
I've talk to 2 customers who wanted the Nest. After I explained that you had to pick between high efficiency, or the Nest, we installed an IAQ.