If the cans of pyro are sealed, they will probably be ok, but if they have been opened, the powder may not be any good. Most of the subs, pyro included, degrade rather quickly after they have been opened.
If you just want to burn some powder, go for it, but don't expect the power or accuracy of new, unopened powder.
However, I would not use it for hunting or for serious shooting.
I would also experiment with some pyro before you run out of black. If the pyro doesn't wanna go off, you may be able to trickle enough black into the flash channel to ignite, or at least blow out any bad, poorly functioning pyro.
You might try a booster charge of 20, or so, gr of black under the pyro to improve ignition until you can get more black...and make sure you clean it well and check the bore after a coupla days. In my experience, residue of pyro is nasty stuff when in contact with steel.
God bless