Here have been using the Pine64 2Gb Arm computer running Ubuntu 16.04 64 bit server now for two years using a microSD card.
Today I would consider the Pine64 (old now) the best Arm based mini pc that will run Ubuntu Server.
The Pine64 with 2Gb of RAM runs circles around any RPi 1-2-3 mostly because it has 2Gb of RAM.
The RPi folks should have added RAM to the RPi3 rather than Wifi/Blue Tooth stuff.
The RPi3 does make for a better KODI box than the Pine64 2Gb or the Rock64 4Gb computers.
I am not knocking the RPi as it is a great little device.
Works fine with my Digi stuff and remote Rpi with a zwave gpio card connected via a poe connection.
Waiting to update to the Rock64 with 4 Gb ram and 16 gb emmc card.
The Rock 64 is the same size as the RPi3 and has a optional wifi add.
Mfg moderators have promised the battery backed up hardware now for over a year with each hardware release.
That said they are still selling last years hardware release.
Posted again on the Rock64 forum about RTC over here:
Tested running xp on the rpi a month ago. It worked but it was way too slow. Thinking I can do this with the Rock64. I am running a lite vb of windows server on the hs3pro box for hs3 sapi speech.
BTW here utilize one Digi 7 port USB hub to multiple Edgeports. Best for you would be utilize 16 serial port 4 usb port digi boxes X3 for 48 serial ports.
I have purchased a few of these for less than $100 used.
For old HS2 here used this stuff plus the Digi USB Anywhere device and Quantech Serial servers.
The company that purchased Quantech is no longer supporting the device and based in the Midwest and is clueless just pushing new stuff.
I've had no issues installing Ubuntu server on the new crop of Intel based mini servers running Windows 10 and Android splitting up the base 32 gb emmc.