From their readme
Q: Can I save my build while working on it?
A: Builds are only saved once published. Work in progress is not saved.
Q: Can I go back after publishing and change my build?
A: No, once your build is published you can't add to it or change it
On the plus side, as if you wouldn't start a mini team to build like 4x4 blocks at once and create something awesome. Like the sydney opera house instead of just a house in the water.. for example (looking at you sydney).