They are not impossible to keep BUT you DO need the right tank, and that is not a regular hobbyist tank! Not a hex, not a bowfront, not something with sand. People don't understand how delicate these are. No undergravel filter, no corners, not even quite the same overflows.....none of these are appropriate for pelagic jellyfish (those that swim around).
You need a Kreisel tank, and there really is no way around it. If you have the proper tank, with a chiller, they are not hard to keep. But please don't do this if you are not willing to make the investment in a Kreisel. You don't necessarily need to spend 1500$, I am sure you can do one DIY, but it is not cheap either.
If it doesn't work, the tanks can be used for seahorses as well, without much trouble, IMO. But they are a challenge to clean tanks, often the siphon is nothing larger than airline tubing and even then, you can damage them.