To be fair, HVAC control is HARD. You are dealing with ASHRAE Thermal Comfort Levels (https://en.wikipedia.org/wiki/Thermal_comfort) that can change from humidity, solar, CO2 levels, the list goes on. Likewise, at least from our perspective, you inherit a lot of the pre-existing issues at a location. Suddenly, if a fan belt snaps and the site gets uncomfortable, you (be it a company or a DIY-er) have to justify it wasn't your fault before moving to actually fixing the issue.
Another problem is getting that sour taste out of the customers' mouth. Even if it isn't your fault, you are on the frontline and they aren't going to be happy with you saying 'Its not my fault.'
Edit: As for ensuring lights are off, the Digi SmartPlug maintains its status when it kills the power. Whatever is plugged into it will be 'off' when you set it to off state, and 'on' when you energize it. I played with their demo set at their conference last year and I hate to say, but it was fun clicking a button on a webpage and seeing a light over some sales rep's head turn off.