The coil lines work best for shooting relatively short distances. The longer the coil, the more likely it will become entangled. Also, when it does become a little tangled, you have to take the time to work it out. If you crank on it when it is knotted, you will put permanent kinks in it.
They work well for shooting relatively small fish under ledges (say less than 10 lbs). The coil line will eventually get stretched and kinked and will need to be replaced like all othe fishing gear. The coil line does slow the shaft down, and I would not want to shoot more than 12 feet with one.
The big advantage is that it allows a much faster and often less tangled re-loading than a typical tethered shaft. They take a little while to get used to using them. Many people absolutely hate them.
When diving New England, shooting under holes with a 3-ft gun, at relatively small fish that are usually no more than 6 feet away, a 10-ft coil line is my prefered set up. I also have used longer coil lines for powerheading large fish with a 5-ft long gun, but in this application the fish never pulls on the coil line hard, since the powerhead will pull out easily.