@Weeg was having problems with his ducks having bumblefoot and I told him about my experience with substrates - that is what he is talking about.
I used to use wood shavings, either pine or aspen. The shavings weren't sharp or large. I found them to be absorbent, soft, and cozy. I often hung out in the barn with my ducks on the shavings. However my poor ducks frequently got case after case of bumblefoot. I eventually asked my avian vet what I could be doing wrong and how I could prevent bumblefoot. She told me very directly that I needed to remove all hard surfaces from coming in contact with my ducks' feet - including wood shavings. I thought she was definitely incorrect and I tried to argue that the wood shavings I get are soft, but she told me that when small delicate duck feet stand on wood chips for hours at a time it is too hard for them. I replaced the shavings with 4-6 inches of straw or hay like she told me to and my ducks haven't had bumblefoot once for years.
Some experienced members here I have spoken with have used wood shavings for years and have not had bumblefoot in their flocks. But in my flock wood shavings were a huge issue. The straw/hay is harder to work with and I have to clean it out more frequently, but for me it is worth it.
People have strong preferences on substrates and everyone should choose the best option for their birds and themselves. There is no one best substrate for everyone.