Hmm. Yeah, I know most people fail to keep them, though I have heard that that's mostly because people (A) don't have a huge tank (B) don't have a sand bed deep enough (C) usually keep a coldwater atlantic horseshoe crab in tropical (warm) water. I have heard that keeping them alive if you have the temperature, food, and sandbed is not that hard. You can actually force feed them by putting them in a separate shallow tank on their back, placing food on their legs, and then waiting for them to consume the food and then excrete their waste, once every three days. At least, that's what researchers do.
Yeah I have heard they are extremely hardy as long as you get the conditions right. My biggest worry isn't keeping them alive (as if I get one, I will MAKE SURE IT STAYS ALIVE GOD ******! lol), but rather that they deplete the live sand. If they're going to kill my live sand I don't want one haha.