It also makes deployments in Ubuntu 18.04 containers fail, because systemd-resolved is already listening on 127.0.0.53:53:

RUNNING HANDLER [unbound : Restart unbound] ***********************************************************************************************************************************************************************************************************************************************************************************
fatal: [controller-dc1r02n01_unbound_container-fbb6fb41]: FAILED! => {"changed": false, "msg": "Unable to restart service unbound: Job for unbound.service failed because the control process exited with error code.\nSee \"systemctl status unbound.service\" and \"journalctl -xe\" for details.\n"}
fatal: [controller-dc1r02n02_unbound_container-dcf7247c]: FAILED! => {"changed": false, "msg": "Unable to restart service unbound: Job for unbound.service failed because the control process exited with error code.\nSee \"systemctl status unbound.service\" and \"journalctl -xe\" for details.\n"}
fatal: [controller-dc1r02n03_unbound_container-b04165bc]: FAILED! => {"changed": false, "msg": "Unable to restart service unbound: Job for unbound.service failed because the control process exited with error code.\nSee \"systemctl status unbound.service\" and \"journalctl -xe\" for details.\n"}

root@controller-dc1r02n01-unbound-container-fbb6fb41:~# journalctl -eu unbound.service
Nov 27 16:06:42 controller-dc1r02n01-unbound-container-fbb6fb41 systemd[1]: Starting Unbound DNS server...
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 package-helper[1047]: /var/lib/unbound/root.key has content
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 package-helper[1047]: success: the anchor is ok
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 unbound[1051]: [1543334803] unbound[1051:0] error: can't bind socket: Address already in use fo
r 0.0.0.0
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 unbound[1051]: [1543334803] unbound[1051:0] fatal error: could not open ports
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 systemd[1]: unbound.service: Main process exited, code=exited, status=1
/FAILURE
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 systemd[1]: unbound.service: Failed with result 'exit-code'.
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 systemd[1]: Failed to start Unbound DNS server.
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 systemd[1]: unbound.service: Service hold-off time over, scheduling restart.
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 systemd[1]: unbound.service: Scheduled restart job, restart counter is at 5.
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 systemd[1]: Stopped Unbound DNS server.
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 systemd[1]: unbound.service: Start request repeated too quickly.
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 systemd[1]: unbound.service: Failed with result 'exit-code'.
Nov 27 16:06:43 controller-dc1r02n01-unbound-container-fbb6fb41 systemd[1]: Failed to start Unbound DNS server.
root@controller-dc1r02n01-unbound-container-fbb6fb41:~# ss -ulpn | grep :53                                                                                                                                                                                                                                                   
UNCONN0      0                           127.0.0.53%lo:53          0.0.0.0:*     users:(("systemd-resolve",pid=50,fd=12))                                                                                                                                                                                                     

Setting

unbound_listen_interface: "{{ ansible_eth1.ipv4.address }}"

in user_variables.yml fixed the issue for me, but it's not very generic of course. If someone has a better idea on how to fix it in general, it would be great!