There are two types of surplus, lot over runs and pull down. A lot of pull down ammo is because it is at the end of it's lifecycle with not enough time remaining to be good before the next scheduled test. So that powder is obviously at the end of its useable lifespan.
There are other reasons for pulling powder, but I assume that pulled powder was pulled for failing a stabilizer content test.
Lot over runs, it is the same as buying new powder. I have no problems with lot over run powders and think they really are a good bargain.
How can you tell the difference between new lot over run and old powder if you are not at the point of origin?