Increasing density too much can indeed lead to blocked up shadows, esp if printing on matte media. You can also get reversals, where the overinking leads to a lighter appearance, instead of a darker one. For example, when you print a gray step wedge, regardless of the exact tonality, one expects the relationships & ordering to be preserved (i.e., the black patch is darker than the next-darkest gray patch). When you put too much ink down, these orderings are not always preserved, and one will see this when the black patch becomes actually lighter than the next-darkest gray patch.