I think Juju (and Charms) really shine more with bare-metal or VM management. We looked into trying to use this for multi-tenant deployment scenarios a while ago (when it was still quite popular in the Ubuntu ecosystem) and found it lacking.

At this point, I think Juju is most likely used in place of other metal or VM provisioning tools (like chef or Ansible) so that you can automatically provision and scale a system as you bring new machines online.