I found a couple of kits in my laundry room that I had bought on clearance at wally-world and then forgot about. They were at least 5 years old when I pulled them out. They had been exposed to extreme temperature changes, as low as 30* and as high as 100*f. The bags of DME were solid bricks and the hops pellets were turned into brown dust. And the Windsor yeast had been expired for 4 years. Rather than just throwing them out, I decided to brew them up and see what happens.
Well after fighting with the malt bricks, I just did two one-gallon batches and tossed the rest. The yeast was a little slow to start, but after about 20 hours, it was bubbling away. The yeast performed very well, and the beer was drinkable in spite of the old hops.
I've never had a pack of dry yest to fail. I've been let down by liquid yeast a couple of times.