I install automation systems and have done a few Nest's at the request of clients.. No problems with them.. The issues I run into everyday are F'ing HVAC guys that can't manage to pull wire that is capable of future upgrades.. If their system requires 2 conductors to operate they pull a cable with two conductors and thats it.. I've asked them what they do if the wire gets cut and why they don't run a 3 or 4 conductor wire just in case.. They look at you like a deer in headlights.. I even run into HVAC guys that flat out tell me you can't run Heat and AC off the same Tstat.... I can go on and on about HVAC guys..

That said, if your system is wired wrong blame the guy that pulled the wire for being to cheap to spend $2 more on a cable with an extra conductor.

As for hydronic/in floor heat.. You shouldn't be changing the temps on those systems throughout the day, it will actually cost you more money to do that, they are meant to stay at a constant temperature to be most efficient.