If the cloud based data and software were open source and available for local backup, then I'd feel more comfortable with it. I'm ok with data that doesn't affect anything being stored out there (local weather data for instance), but I'm not going to have my HA server on such a system.
Point in case, I was looking at a weather station a few years ago. The best option for a wireless setup that appeared to be affordable was the Acurite weather gateway. That approach included a thermohygrometer for external use, a receiver that included an LCD display with indoor temp/humidity, and the network plug "weather gateway". This device received the signal from the outdoor unit (but not the indoor) and uploaded the data to Acurite's cloud. I bought the unit and it worked, but then I found out that it was proprietary and would not allow data to be stored locally nor sent to a 3rd party server (Weather Underground, Pachube, etc). So, I took it back. I didn't trust that Acurite wouldn't change the service to be paid, cancel it, or even sell my data without consent. If WU, Pachube, etc go under then I might lose the data stored there. But, since I know that is a possibility, I'd be careful to keep local backups of whatever I need. The Acurite approach wouldn't allow for local backup since it doesn't expose the data locally (without somehow hacking the gateway or intercepting the transmission).