Actually, I have a 'smart' thermostat. I asked a bunch of companies what would happen to their device if their servers stopped working. Nest does almost nothing without the 'cloud'. Hive (via British Gas) never gave me a straight answer. I asked repeatedly, but all they'd say is that I needn't worry, they're not going anywhere and so the servers would always be there. AFAIK, it turns into an ordinary bi-metalic strip type thermostat if there's no cloud.
I ended up with a Heat Genius system because it carries on working (albeit without remote access). It's got a 'cloud' connection, but it's optional. I can either use uPNP, or else I have to open up firewall ports to let the cloud (and support) in. If I don't do so, then it all just works LAN-only.
The system is a hub (with a raspberry pi in it, I believe), and some z-wave radiator valves, UFH valve switches and a few other bits. In theory they're all hackable, although I seriously doubt you could do much that way - the comms between hub and device isn't really up to it. If the actual hub box got p0wned, it'd be a pain in the arse (about £200 to replace, although I'd probably argue enough to get one for free). In the interim period, it's possible to turn it off and let each of the radiator valves work manually (they have some little buttons that set the target temperature). I don't think our underfloor would work at all though.
So... why bother with any of this? Well, when it's working properly, it's actually very good at controlling the temperature in the house. It uses some fancy logic to just about heat the room to the target temperature without over-shooting it. It also maintains temperature very well. My memory of physics suggests this should be cheaper to run than more traditional setups (although I don't have any decent facts either way). If I'm honest, it's got quite a few rough edges, and some annoying bugs. Having said that, I can't fault their support, so getting to the bottom of what's going on pretty quickly.