I see you don't remember Sasuke's fight against the Raikage, and Madara's fight against the Gokage (Tsunade was able to crack Madara's smaller Susano'o).
IMO:
Susanoo has great force but if it is being attacked with greater force, it will crack. Also Danzou used wind style jutsu to penetrate Sasuke's Susanoo. Because Danzou figured it out that Sasuke's Susanoo was bit weaker at back mid rib point and Wind style jutsus are the best jutsus to penetrate among all elemental jutsus.
and i think same thing goes for yata mirror(every jutsu has weakness)
so yata mirror cant cover whole susanoo,so you can attack from behind or from side to attack itachi,you dont have to attack on yata mirror