I haven't had issues using old milsurp ammo.
Just cracked open my last case of Portuguese 7.62 NATO dated 1985, other than the battle packs smelling like mildew it's shot fine.
Same as all the other I have used, even the ones not sealed in heavy PVC.

In the eaarly 2000's I was getting OHV 7.62x51 Indian ammo, 400 round cans dated mid 1970's, looked like they had been on the bottom of the Ganges.
The bandos were a bit musty but no mold. It shot fine, kinda smokey though.
There were a couple of cans that looked really bad and had to be beaten open with a hammer, thought for sure I wasted $25 on those but the ammo inside was GTG.