None of it's particularly hard. I wrote a node.js server that serves a webpage for the dashboard, and passes messages back and forth between that webpage and the various devices.
Belkin WeMo switches and motion sensors are UPnP. Discovery can be flaky sometimes, otherwise they work well and there's plenty of sample code out there for UPnP and WeMo specifically.
The garage doors are Chamberlain MyQ, a proprietary website/app with no public API, but the website talks to a JSON API that was simple enough to figure out with Chrome's network inspector. I probably wouldn't buy MyQ stuff again if I were starting over just because of how closed it is. They don't want to integrate with anything else.
The Nest thermostat has a JSON API, also unofficial though an official one is in beta testing. There's a node package for that, so plug-and-play for me.
The rest of my stuff is Z-wave (short range 900MHz radio), which you need a hub for, to bridge that network to the home IP network. I got a Vera Lite for that as it's the best hub I found that has no subscription fees, is entirely self-hosted (there's an optional cloud component for remote access and setting backup without configuring your firewall), and exposes a REST API. That was the easiest thing to code against -- to lock/unlock the deadbolt on my front door for example, I just GET a URL with the device ID and command in the query string.