The obvious reason why Greece is called rugged is because of its mountainous terrain. Mountains are commonplace throughout Greece, with the Olympus Range being the most prominent. Another reason the nation would be considered rugged is because of the severe lack of arable cropland. Less than a third of the nation's land is suitable for growing crops. Speaking of the land, reason three would be that Greece is lacking significant natural resources, especially valuable ones like energy sources or precious metals. Lastly, Greece is subject to a number of natural disasters such as droughts, wildfires, and earthquakes.
For bonus, a fifth reason Greece is considered rugged is the numerous islands that make up the nation. The nation is comprised of over seven thousand island, making travel and trade more difficult than if it were splintered into thousands of islands.