The thing I would be worried about is that when wet, fallen leaves will retain moisture and rot. Chicken poop is about 75% moisture. Even if it seems dry on top, it can be wet and rotting in the layers below. The main thing I would be worried about (aside from the bugs and creepy crawlies) is when it starts rotting, will it impact the chickens' sensitive respiratory system?
If you are to use this as coop bedding, I would definitely completely change it often and probably not use a deep litter method. It would also depend on what type of floor your coop has. If your coop has a ground/dirt floor, it may be great. If you have a wood floor, this bedding may rot the floor out.
Think of going hiking in dense woods in the middle of fall. If you dig down into the leaves, it will always be very moist and the layers will be rotting.