Good dry cottonwood bark will light up so fast you can barely get it out of your hands before it burns you, which is good.
On the other hand, I had trouble using cottonwood bark I carried in a plastic baggy one time, and I think that may have been because there was a small amount of condensation in the baggie. The bark looked and felt dry, but just didn't want to catch fire.
I'm thinking maybe well shredded and dry juniper bark, with it's natural oils may be the better tinder of the two, in the case of too much humidity. I have not tested that assumption yet, though.
The forests are closed to fires around here now, which is as early as I ever remember them enacting fire restrictions. It may be a while before I test my theory and replicate the test under some humid conditions.