Most of them are not scams, but you do have to be careful. Mostly, they are looking for equity. So no, even most of their signs say they will buy any house at any price, it's not true. For example, if you owe 200K on your house and it can only sell in today's market for 180K then most likely you'll be out of luck.
However, if someone is in that position they can often save their home but it might mean a decrease in lifestyle. For example, you convert other rooms or space into livable space and rent out the bedrooms. Or even move into an apartment and rent the entire home.
Be careful about signing anything that you don't fully understand and most of all, listen to your gut feeling when dealing with any investors. Good ones will explain exactly what they do and how much money they need to make in order to do a deal with you.