NOT a lawyer, but I think that's wrong. The reverse mortgage means that if they had one, and the heirs wanted to keep the house, they would have to basically buy back the house from the company. Heirs are given the option to do this, but it's not mandatory they do so. I've seen a few stories lately that said some of these companies were misleading heirs and foreclosing on the houses without them really understanding all of the options, but I've also read that RMs are on a decline lately...
But there is no way they could decide that the house isn't worth what they paid so they're going to take it out of the heirs - I can't image that is in any way legally enforceable. The heir didn't enter into a legal contract and a relative can't represent them or bind them into an agreement without their knowledge or consent. You just don't get to keep the house.
And as far as what debts can be passed on to heirs, well, none that I know of. Any money owed at the time of death is then taken from any existing estate. If there is property (less some personal items I believe that can be held out in some cases but again, I'm NOT a lawyer) then they can force a sell-off to satisfy the debts, but once the money/property is gone, that's it for what they can legally tap. Which means the heirs themselves might be left with nothing, and might have to pay funeral expenses if there isn't enough in the estate or other plans already made, but they can't be held liable for the deceased person's debts unless they co-signed something legally obligating them.
I know when my dad died, the state required us to publish a notice in the paper to alert any creditors. They had 6 months from the date of that notice (whether anyone saw it or not) to file a claim against the estate, and after that point in time, they're out of luck no matter what the circumstances. I'm pretty sure that's not common, but if there were any obscure debts we didn't know about, too bad, so sad.