A few things about brittle/serpentstars:
1) there is no biological difference between "brittle" and "serpent" stars. It is a distinction made in this hobby only and has no scientific significance.
2) Brittlestars will not generally come out during the day. This is normal behavior, and very healthy. The hide under or around rockwork. To get them out during the day, you need to add food, or dim the lights, but they stay hidden as visually acute fish (meaning day active) are their primary predators.
3) Brittlestars are not coral eaters. Perhaps if a piece is dying...but they are not like certain seastars and do not generally feed on corals...however, they may live on a coral. A distinction needs to be made there. Some are symbionts on corals...and in some cases if the coral doesn't have them, they often die (see Astrobrachion constrictum...think I spelled that right)
4) The green brittlestar is very large, as people mention. It is a known predator in the wild, but this does not mean that it will be in captivity. I reckon that considering the number of people who keep them, few have problems. Other people have problems with different species, not known predators in the wild. Every tank is different, every animal is different, and no generalizations can be made.
5) Brittlestars should be spot fed. Our tanks are usually heavy on the "clean up crew' and often light on feeding (or fed a lot of liquid foods). I recommend spot feeding.
6) If you have issues with losing shrimp or small fish, I would not get one. But then you have those issues with many animals. Not all brittlestars, even all greens, are predatory. But you must put yourself in the animals position. The rule of life is to grow large and reproduce...to do that you must eat. Few animals will pass up a meal if they need it. And in this hobby, that means eating things you may have paid for. That is the drawback. We want "natural" ecosystems but often cringe when this happens.
The important thing is to ask why you want it? It is not, IMO, good for sand sifting or cleaning detritus. But it is a very cool and interesting animal that is extremely common on natural coral reefs.