I have Vox on my 2012 MacBook Pro (OS Sierra), on a refurbished 2012 iMac, and on mu Apple iPhone SE (currently 12.1.2). I also have been using the Vox app on my phone with Apple CarPlay in a 2018 Golf SE. I love this software! It makes a difference!
So far my Music Library (nearly 400 Gbs) is all in the Vox Cloud, and it is all FLAC files. I have no problems with cover art on my Macs or my iPhone because I embed the cover art in the music before I upload it, using the XLD app. I also use “iso2dsd” for OSX version 6, to convert DSD files to FLAC files.