Green or wetter wood, chips better than old dry hard wood.
I have to sharpen my blades on the chipper a lot more when chipping old dry wood. Green and wet wood has it's own problems. It's more likely to gum up the flywheel with pitch and tar, which requires more maintenance.
I have such a variety of stuff to be chipped, I find in the end...it dosen't matter. I do like chipping spruce and fir...the smell is so nice. Like a fresh cut Christmas tree.
I have mostly pine and Douglas Fir branches that are mixed with old and new. My question had to do with water soaked from constant rain. Sorry, I should have been more clear.
Wet will be harder on you -- you'll get soaked handling wet branches -- but makes no real difference to the chipper. A few years ago I felled a beech tree one day and chipped the branches the next day. Unfortunately it rained overnight. I was soaked to the bone after feeding all those wet branches through the chipper.