The argument can be made that militias are, in some cases, harder to fight than conventional armies. When the enemy is comprised of un-uniformed individuals who operate out of homes and flow from the crowds of innocent bystanders, instead of a unified, uniformed army operating out of defined bases with attack plans and everything else that comes with the organization of a national army, I would say that the militia would be a tougher enemy for a country not involved in total war or a country concerned over its civilian casualties.
History has shown time and time again that organized armies have a very hard time beating militias and stateless factions, just remember Russia in Afghanistan, the United States in Vietnam, the United States in Iraq and Afghanistan, and even think back to Great Britain in the American colonies. These are/were the world's most powerful militaries.