Normally you also make payments towards the principal and build equity. As I understand, most of these buy to let loans actually only have you pay interest so you'll never own the property. If the value even after 20 or 30 years drops below the initial value, you're in the negative and need to pay up the difference if you can't make payments anymore. Whereas with a normal mortgage once you've paid it off, fluctuating values can't put you in severe financial trouble.