I finally have some time to kick the tires on Zentyal 5.0, so I am starting with a fresh install. Except for a couple issues so far so good. The first issue is the GUI wouldn't come up, and after perusing the X log for whatever reason no video drivers were loaded. "apt-get install xserver-xorg-video-fbdev" fixed that problem. The second issue is I accidentally entered the wrong host name when installing, and I was getting an error with the webmin trying change it. It was blowing up enforcing the timezone because /etc/localtime is now a link instead of an actual file. So "rm /etc/localtime;cp /usr/share/zoneinfo/America/Chicago /etc/localtime" fixed that issue.
The plan is to upgrade my personal server, run it for a few days, and see how it goes. If that goes well then upgrade my business server and run that for a couple weeks. And if THAT goes well for a couple weeks I will start upgrading customer servers. It will be great to have all my servers based on Ubuntu 16.04, thanks Zentyal folks.
PS Feature Request: Integrate the certificate authority with LetsEncrypt. That would eliminate the bulk of the customization I do at this point.