Dirty Italian
Well, firstly, it's extremely difficult to win by playing defensively. Methods to counter both a large group of defenders during a siege and strong walls have long been developed. The most effective is an economic blockade. The Dwarf will have to fight in the field to avoid being strangled by a resource blockade.
Secondly, the Dwarves' initiative still leaves much to be desired. Yes, they are strong and tough, but attack is not their style. Therefore, high-initiative attacking races can easily overpower them. And every race will find something to counter them. Otherwise, everyone would only be shouting about "imba Dwarves."