What you're seing now is just an alpha release of StarMap and the Nebula platform. It's meant as an early preview of what's possible with the platform, but there's certainly a lot of work to still do. Beyond just building more applications, there are a few major features that are really missing:
You can read more about our current security limitations here.
TL;DR: Your data is safe enough for now, and we're hard at work on the technical hurdles to get it totally secure.
As it turns out, it's a hard problem to get the state of all your data synchronized across multiple devices that may or may not be connected to the network at any given time. While we do have a lot of support for synchronizing your files across multiple devices in place, it's not currently in a state where we feel comfortable having people rely upon it. Until this is ready, your clouds will unfortunately be limited to one device at a time. Don't worry though, once we get support for multiple hosts ready, you'll be able to mirror any clouds you've already created to new devices.