Of course, the main reason for using ballistic missiles is because they are difficult to intercept. In theory, a cruise missile is detectable, and as vulnerable as an aircraft. But in practice, South Korea has the technology, and capability, to build a cruise missile that can fly low (under 200 feet) and avoid detection by radar. South Korean knows a lot about the North Korean air defense system, which is decidedly low tech (although quite massive) compared to what South Korea has. Actually, the low tech aspect (lots of human spotters and elderly anti-aircraft guns) is the biggest danger the Cheon Ryong will face heading north. But the South Koreans know that, and the Cheon Ryong can carry a cluster bomb payload for attacking the anti-aircraft guns. Still, while the Cheon Ryong may be able to avoid missiles, it's going to have a harder time with all those bullets. But a cruise missile can be more accurate than a ballistic missile, and come in a different angles. That will be useful in taking out the many underground bunkers up north, or at least the entrances.
South Korea announced that it has developed a cruise missile, the "Cheon Ryong," that is similar to the American Tomahawk, but with a 500 kilometer range and a half ton warhead. This was not unexpected. The U.S. had to apply a lot of pressure on South Korea in the 1990s, to stop production of ballistic missiles. The South Koreans eventually backed off on this, despite the hundreds of ballistic missiles North Korea had built, and aimed at them. But the shift in policy wasn't completely because of American pressure. The South Koreans realized that cruise missiles would be cheaper, and just as effective, as ballistic missiles. South Korea had the technology to build good cruise missiles, and a lot of them. Not a lot of details were released on the "Cheon Ryong," but it is small enough to be fired from a torpedo tube. That means a 17-19 foot missile, with a diameter of about 500mm, and weighing about 1.2 tons. Such a missile could be built for about $1-2 million each, which enables you to build about five cruise missiles for the price of one ballistic missile.