After hours of thinking about this, I really cant come up with a reason why a forest would ever be a disadvantage as forests are home to many animals our uses of wood comes from the trees that make up a Forest.
A Forest stops land erosion and helps make oxygen. Maybe a Forest fire could be construed a disadvantage but fires aren't the forests fault and fires actually thin out forests for future growth, One disadvantage could be, if you live in or by one, would be to rake up all the leaves that fall in the fall.