I ran up a Symbiosis VM because I wanted a very quick email setup with spam and antivirus built in. It was excellent for that and saved me a lot of time. I then set about migrating several web sites and applications from my old Bytemark VM.
In general I've been impressed with Symbiosis which is performing well; everything I wanted has been achieved so far, with a little help from the forums here.
I particularly like the Let's Encrypt integration which saves time and money. The easy email and web site configuration is great too.
What could be improved? A few thoughts.
Security - personally I think the default should be that all servers are encrypted and passwords never sent over plain text. FTP seems to be unencrypted by default, except for the admin user.
By default the /stats page in a web site is open to the world. I doubt many users want this and I think either stats should be disabled by default, or else offered in a password-protected page.
Documentation - the docs are helpful but I could have done with more guidance on the way Symbiosis works and what happens when you change config files manually. In particular I found the DNS system confusing.
Bugs - haven't found much but I did trip things up when I changed a web site from an alias to standalone. I needed to manually move it into sites-enabled.