I've bought one, and it seems to be working mostly OK so far.
The Armbian installation was straightforward. It would be nice to have a summary of how the default installation differs from a regular Debian installation, though.
Its NFS server seems to be working OK.
Setting at Samba for Mac Time Machine has been a bit more problematic. Having copied&pasted some example smb.conf files from random websites I got something that worked... until it stopped working, after about 200 MB of the initial 200 GB backup, with no useful errors logged anywhere. On a hunch I've tried disabling IPv6, and it has (so far!) got 1.6 GB through the same initial backup, hmmm. This probably isn't the best place to ask about this, but if anyone has experience with Time Machine using Samba (rather than netatalk) please let me know!
Thanks again for the recommendations.