The biggest disadvantage I have heard is that it is not consistent from lot to lot so you may have to work up new loads whenever you switch to a different lot.
For accuracy, the appropriate bullet for the rate of twist is far more important than the powder.
I use mil powder in all my plinking ammo. It's cheap, relatively clean, and it works. I have no complaints - neither has anyone that's shot my ammo.
For .223, Wc844 mil is consistent enough to load batch to batch with very little change in velocity. It's not match powder by any stretch, but you won't need to work your loads up every time you get a new jug.
I've been told that surplus power won't give great accuracy, it's cheap & made for machine gun ammo
Has anyone noticed any difference between "new" surplus powder and "pull down" surplus? The pull down is sifted, so whats the downside?