I tried to upgrade via the instructions doing a git pull and then running ansible again and it totally broke my site with a server error message. I ended up reverting back to 0.17.4.
I just upgraded via Ansible and the only issue I ran into is that on 0.18.0 you can't enable Federation and Private Instance at the same time. I had both enabled on 0.17.4 and when I upgraded, the Lemmy docker wouldn't start because both options can't be enabled together anymore. I had to disable Private Instance directly on the database to get my instance up and running again.