On the issue of Back Cover I do have to agree with both DarkosOverlord and Audo, as overblown expectations like i.e. the Foretellers faces being revealed or an definite answer to the Traitor or the nature of the mysteries given in X[chi] certainly do play a role to an extent.
If one thinks about it, revealing one definite Traitor or too many of the questions raised in X[chi]/Unchained would take out all the mystery and ominuousness on the further developments in Unchained X and most likely also on parts of KH III, not to mention that there cannot be a single Traitor revealed among the five Unions (if there is one, that is) while Unchained is still running as this would automatically make the Traitor's union into a pariah and would most likely cost it most of its members.
On the other hand though I agree on the criticism points that Back Cover certainly was too short overall and ended way too abruptly with a heavily unsatisfying note, Skuld didn't even appear and Ephemer had "just" a glorified cameo (which was awesome and diabetes-inducing nonetheless I'd say), but it certainly cannot be labeled a "total waste of time" or failure as it did really a good job on introducing the Foretellers, their personalities, motivations and character traits as well as the Master of Masters.
Yea, they certainly did several stupid things and eventually caused an apocalyptic war (seriously, while first watching BC I actually mentally wanted Ava to bash Gula over the head for suggesting such an obvious dangerous and stupid course of action), but unlike some other entiries in the series their "stupidity" and facepalm-worthy decisions were actually placed in a reasonable and understandable context, not dictated by plot-induced stupidity or for the sake of yet another "surprise" twist.
It does look like a deliberate setup and maybe it is just like that, but from what the movie does show us is that the Foretellers apparently really love and trust their Master so him setting them up and possibly deliberately coaxing them into causing what they actually want to prevent is something that would not even cross their minds in their wildest dreams.
Even Aced, who is outwardly the most critical of the Master's teachings and suggests that in order to save the world, they have to defy them, only ever suggests that the Master is mistaken in some of his teachings and orders he gave them, never that he actually set them up.
You could even project this somewhat on the present day cast of characters (only as a comparison, not relating to any rebirth/another set of clones-theories), replacing the Master of Masters with Yen Sid and the Foretellers with SRK, TAV and possibly Mickey, Donald and Goofy.
Yen Sid is a wise and respected sorcerer and teacher, so would any of our present day characters entertain the thought he could set them up deliberately to screw everything up?
Same most likely goes for the Foretellers and their Master.