For new player and moderate players, the concept for the challenge and mission can be better.  I did come to frustration in which I did do the mission or challenge but didn't get the reward.  Why?  Because it is either not in chronological order or the part hasn't being offered yet.  It is better if Plarium presents the players the whole list/part and let us finish anytime during the game as fit without having to repeat because the reward has been given out yet.