Yeah it's okay but it's not as good as it seems. There's no reason for you to be using armour after let's say 21122 since he's already -1 so you can poke afterwards with an 8f poke or less (MU specific). If he cancels into EX Sand Grenade and you're trying to armour you get hit. It's a pointless risk to take. Not to mention Sos2 whiffs on crouch block and since you should be crouching against Erron you can whiff-punish on reaction.
It's not even a mindgame between getting hit by Sos2 & taking the chip of Sos3 since you can blow him up with armour every time on the last hit of Sos3.