Yes I have looked into drop bear and it mostly works. Unfortunately the Official Ubuntu Package does not come with the scp command so you won't be able to securely copy files from within the guest machine to another host.
Please see the relevant issue on Launchpad:
https://bugs.launchpad.net/ubuntu/+source/dropbear/+bug/472264
This is might sound like a minor issue, however when you turn the virtual machine into a Vagrant Box, Vagrant will complain that the guest OS does not have SCP capabilities and will cancel the provisioning. Having said that, I'd rather prefer the OpenSSH server package over Dropbear for the reasons above, at least until Dropbear will support SCP as well.