Decided to try the magic-device-tool to see if that'd make any difference. Nope!
Pretty much the same output (below) as flashing via terminal.
=======
Created filesystem with 11/51296 inodes and 6651/204800 blocks
target reported max download size of 134217728 bytes
erasing 'userdata'...
FAILED (remote: unknown command)
finished. total time: 0.002s
Creating filesystem with parameters:
Size: 4194304000
Block size: 4096
Blocks per group: 32768
Inodes per group: 8000
Inode size: 256
Journal blocks: 16000
Label:
Blocks: 1024000
Block groups: 32
Reserved block group size: 255
Created filesystem with 11/256000 inodes and 34123/1024000 blocks
target reported max download size of 134217728 bytes
erasing 'system'...
FAILED (remote: unknown command)
finished. total time: 0.002s
erasing 'recovery'...
FAILED (remote: unknown command)
finished. total time: 0.002s
rebooting into bootloader...
OKAY [ 0.002s]
finished. total time: 0.253s
Please wait
2017/05/27 21:13:42 Device is |frieza|
2017/05/27 21:13:42 Flashing version 5 from ubports-touch/legacy channel and server http://system-image.ubports.com to device frieza
2017/05/27 21:14:06 Cache formatting was not successful, flashing may fail, check your partitions on device
2017/05/27 21:14:17 Start pushing /home/ronnie/.cache/ubuntuimages/ubports-touch/legacy/frieza/version-5.tar.xz to device
2017/05/27 21:14:17 Start pushing /home/ronnie/.cache/ubuntuimages/pool/keyring-4c4e7ef380ebcfa2c31084efa199138e93bfed8fc58aa3eb06bdf75a78af9b57.tar.xz to device
2017/05/27 21:14:17 Start pushing /home/ronnie/.cache/ubuntuimages/gpg/image-master.tar.xz to device
2017/05/27 21:14:17 Start pushing /home/ronnie/.cache/ubuntuimages/pool/ubports-f683ce0eac8665810600110821a12e5ad615e5d509507c90cea46a0f254adc23.tar.xz to device
2017/05/27 21:14:17 Start pushing /home/ronnie/.cache/ubuntuimages/gpg/image-signing.tar.xz to device
2017/05/27 21:14:17 Start pushing /home/ronnie/.cache/ubuntuimages/pool/device-6d8854a9ee3fd34df55940ae6e19a6968b14418a7fd01fbf1e17624d0129ef23.tar.xz to device
2017/05/27 21:14:17 Done pushing /home/ronnie/.cache/ubuntuimages/ubports-touch/legacy/frieza/version-5.tar.xz to device
2017/05/27 21:14:18 Done pushing /home/ronnie/.cache/ubuntuimages/pool/keyring-4c4e7ef380ebcfa2c31084efa199138e93bfed8fc58aa3eb06bdf75a78af9b57.tar.xz to device
2017/05/27 21:14:18 Done pushing /home/ronnie/.cache/ubuntuimages/gpg/image-master.tar.xz to device
2017/05/27 21:14:18 Done pushing /home/ronnie/.cache/ubuntuimages/gpg/image-signing.tar.xz to device
2017/05/27 21:14:44 Done pushing /home/ronnie/.cache/ubuntuimages/pool/device-6d8854a9ee3fd34df55940ae6e19a6968b14418a7fd01fbf1e17624d0129ef23.tar.xz to device
2017/05/27 21:15:20 Done pushing /home/ronnie/.cache/ubuntuimages/pool/ubports-f683ce0eac8665810600110821a12e5ad615e5d509507c90cea46a0f254adc23.tar.xz to device
2017/05/27 21:15:20 Created ubuntu_command: /home/ronnie/.cache/ubuntuimages/ubuntu_commands360842843
2017/05/27 21:15:21 Rebooting into recovery to flash
Wait until it reboots on its own!
Do not reboot manually!
Cleaning up..
Exiting script. Bye Bye
======
And, from there, even after waiting for 25 minutes I see the infamous yellow 'Installing Ubuntu update' text. 

If I choose 'reboot system now' it boots up, again, back to a fresh install of OTA-15!