I love them and always had great success with them, but as the TITLE reads Fish NOT for the beginner. Ribbon Eels are not a fish with a great success rate so why recommend them for a beginner that is just starting out in the hobby when there are FAR easier Eels such as the Snowflake, Chainlink, Zebra, etc? Lately when I do see Ribbons (which is rare) they are over 70 dollars (back when I kept them they were around 30 for a black or blue), so IMHO it will be a 70 dollar mistake for a beginner and a loss of a Ribbon Eel. They are hard to get to eat and even if you surpass that hurdle they are SO tiny and thin they can get out of most holes in the lid. JMHO!
One fish definitely not for beginners (may already been stated) is the Boxfish. Poisonous and semi aggressive also expensive experts shouldn't even take this one home. They also need special foods that a beginner wouldn't like to get. They are hard to breed and will nibble on tube-worms. They also need a large tank. Definitely not for beginners.
I'm by no means an expert on SW tanks. However, I have a Mandarin Dragonet that is doing very well in my 90g tank. It's actually improved its health since it first arrived at my home. It's fatter and swims frequently. Though I may have lucked out because mine has no aversion to frozen brine shrimp (must be thawed first). I do not think these fish are difficult to keep if the hobbyist does proper research before purchasing one and has a tank of the right size with plenty of LR. Of course, imo every fish should be researched before adding them to the tank.