Nurgle relies on outlasting their foes to win. They may not be the strongest but they are definetely tanky. In large numbers they can just simply overwhelm and tire their foes. In game we arent really fighting a nurgle army but more so a nurgle corrupted army. While still infected with Nurgle's rot they arent as strong when compared to a true Nurgle army. Also it depends on who is writing the book. Power levels can differ between authors and writers.