Going to chime into this topic and give my brief two cents.
I feel like Back Cover was actually pretty good for the most part. I really liked that they touched upon the Foretellers and gave them unique personalities. I also really liked the comic relief with the Master of Masters. The soundtrack was also great, but only a few tracks have been uploaded somewhere from it.
Anyway...back the the point. The character development was pretty good. I wished they would've shown a little more of Ephemer and included Skuld in the mix. Perhaps, they should've waited to release Back Cover till post Union-Cross. It also would've given them the excuse to release 3D renders of the Dandelions and new Foretellers. Overall, I liked the movie, but felt like it was missing some tidbits here and there.