I didn't find it to be an issue. Nothing that sunlight didn't get out and I didn't use liners. They are not as dark once they are on solids (I'm assuming your baby isn't as they are dark and runny).
If you are worried you could try using some cloth liners in the sposie as a test maybe.
We had constipation issues before going on neocate so not sure if it normally makes things runny. For us he did get runnier but that was a good thing.
Yeah the smell is awful isn't it, and the neocate pukes

It does get better with more solids/less milk.