I got OnlyOffice working on Nextcloud 19.0.1 and I love it!
I used the Docker install method as my Webserver is Ubuntu 18.04 LTS/Apache. I have memcache.local configured for my Nextcloud instance and no warnings or suggestions in the admin/settings.
My only issue is performance. My server is very adequately provisioned with 8GB RAM and SSD-Backed SAN storage.
Can anyone suggest a method of improving speed of OnlyOffice document opening/editing?