The Phenols (oils) in softwoods (pine and cedar) can
cause upper respiratory problems in animals, however it isn't guarenteed
that they will or will not.
Hairless rats are affected by nearly anything, and the only bedding I found that they don't have a negative reaction to is newspaper, but it needs to be cleaned more often.
Then of course you have human allergies and reactions. I can't touch any kind of softwood without breaking out in hives and struggling to breathe
I think it's just important to realize that there is the potential
for softwoods to cause problems in animals.