it's very easy to learn the mechanics since it's basic combo is unlocked from start
the fun part in combat is to custom your active skills, special moves with some effects like range attack up, defense up, invisible, area attack, rush attack, etc
is better than Umbral Star in every way except maybe OST
about difficult, in first it seems rather easy since you can repeat some chapters of the story line and in some chapters the lvl required is the same as it's similars
but later the dificult starts to elevate and becomes a good challenge, and if you don't want to level up your servants is the Money is Power System to lvl up servants to the highest level possible, the max level possible is the lvl of you most leveled servant
to say an example, Tamamo is lvl 1 and Nero is the most leveled servant (lvl 30), the max level possible is 30 for Nero
half a month later but well... better later than never