Let me give you my experience with these caps and rotors.
I had gotten tired of the original plastic POS distributor. If for no other problem then the dreaded cap screw holes that crack. So I bought a nice Summit aluminum unit and it worked well for several years. Keep in mind I live in the second worst area for humidity. The first being southern gulf states.
Humidity never EVER played a roll in these POS caps. I had gotten to the point where my caps were destroyed in 300 miles. I took a trip to Jersey which for me is 6 hours and 300 miles. I had to put a new cap on and take one with me to change out on the way back. And I did once.
A couple years ago I finally had enough of the problem and I removed the distributor and found the 2 vent holes were completely closed from flaky aluminum corrosion. I did not like that the vent holes in the aluminum distributor were right angle so I drilled straight down to clear them and open a good hole in both of them.
Now my caps last 10's of thousands of miles. So no, humidity did not play a role.
Now let me tell you about the caps and what happens. Once when I had a bad miss show up, I bought 2 new caps and one of them was a Delco cap. Both new caps had a miss right out of the box. Look at the photo below. I drew an arrow to the area that is a problem for ALL manufacturers. I know this area is a problem continually because if you search the internet for 5.7 Vortec miss-fires, you will come across dozens of people that have cylinder 3 miss-fire.
Although I don't have a x-ray of the area in the arrow, my guess is that when they lay the conductor in the mold, either they lay it too close to the center contact. Or the injection process pushes the conductor too close to the center.
Incidentally these people (on the internet) have stated during the bad miss-fire event, if they pulled the cylinder 3 plug wire off the plug, The miss-fire changed dramatically. The area shown in the arrow is the number 3 contact.
There is a TSB on the original distributors to punch out the screen in the vents. So that backs up what I experienced with caps going bad quickly from lack of venting.
Oh and one last thing. When I had the vent problem, my contacts where 100x's worse than those in your photo. The white corrosion was a couple mm thick.
If you want a cap that lasts way WAY longer than the usual aluminum contact caps (like in your photo) get the ones with brass contacts. Like the MSD 8017 available everywhere including O'Reilly's. There are a couple other mftr's of brass caps too.