Did some digging
https://community.ubnt.com/t5/UniFi-Updates-Blog/UniFi-SDN-Controller-5-8-28-Stable-has-been-released/ba-p/2449036
Important Notes:
Linux systems must be running a version of MongoDB prior to 3.6.x. We recommend 3.4.x. This is most likely to be an issue on Ubuntu 18.04 LTS, as it currently offers MongoDB 3.6.x. >
The initial database migration will take longer than normal. It is expected to see mongo using most, if not all, of the available CPU cycles during this process. Please be patient, this process could easily take 15+ minutes, depending on the amount of historical stats, as well as the system specs. As always, err on the side of caution, and make a backup before upgrading.
The controller will not start if it is set to bind to a privileged port (<1024), as it now runs as a non-root user.
If your controller is running on a UniFi Cloud Key (UCK), make sure it is on firmware 0.6.4 or later, otherwise the controller will not start. This firmware is available via the normal upgrade mechanism found in the controller or it’s local management page. Make sure to make a backup before upgrading the UCK firmware, as you’ll need it to restore after, and it’s good to have a backup on hand before any controller upgrade.
Support for PicoM2 and 1st gen AC models was dropped in 5.7.x. Please see our announcement HERE.
As of release branch 5.7.x we’ve made some important changes to the Wireless Uplink feature (feature details HERE).
We removed the “Enable automatic uplink failover” from wireless uplinks as it is no longer needed.
We added the ability to opt an AP in or out of wireless uplinking to another AP. This is done by checking the “Allow meshing to another access point” option found under device properties>Config>Wireless Uplink.
This option should be disabled on wired APs, but is required to be enabled on wireless APs.
If you disable this option on a downlink/wireless AP, then your AP will be disconnected from the network and require further action (including physical access).
If upgrading from 5.6>5.7 then this already be disabled on wired APs, but if upgrading from an earlier 5.7. release then this may be enabled on wired APs.
Again, this option should be enabled on downlink/wireless UAPs only. It should not be enabled on wired UAPs.
We added the ability to set uplink priorties. This allows you to define the preferred uplinks for a downlink/wireless AP.
We’ve found that some migrations from 5.6 to 5.7 have resulted in Fast Roaming being enabled, which is not our recommendation. If you see this, feel free to disable Fast Roaming on all Wireless Networks.
Fixed IPs (DHCP reservations) are now required to use unique IPs. The same IP cannot be assigned to more than one device. For configurations that already contain duplicates, only the most recently active device will have its fixed IP provisioned to USG. The controller’s server.log will contain a log message skip provisioning duplicate fixed IP for user[] indicating which was omitted where there are conflicts.
Custom port profiles on AC-IW/IW-Pro are not supported until the 5.9.x release branch.
UniFi - How to Install & Update via APT on Debian or Ubuntu
Biggest thing I see is this
UniFi - Recommended Minimum System Requirements
Operating System:
Ubuntu Desktop / Server 14.04 or 16.04 (recommended)
Debian 7 “Wheezy”, 8 “Jessie”, 9 “Stretch” (recommended)
macOS Mavericks 10.9, 10.10 Yosemite, 10.11 El Capitan, 10.12 Sierra, 10.13 High Sierra (10.13 recommended)
Windows 7 or Windows 10 (recommended)
Windows Server 2008 or 2016 (recommended)
CPU: x86-64 Processor (Intel / AMD x64 Processors)*
RAM: 2GB*
Network: 100Mbps Wired Ethernet*
HDD: Minimum 10GB free (20GB or more preferred)*
Java: Java Runtime Environment (JRE) 8
Web Browser: Google Chrome
MongoDB: version 3.2 or later (Mongo is offered bundled: default bundled is 2.4.14 (for macOS and Windows only)).
Don’t think there is an ARM port
more poking around, I found this
https://station.eciton.net/