How to be sure you will pay me back with full interests? With banks, the solution is easy, the bank seizes the house and you can't do something. But the average Joe hasn't the same 'power' than a bank to collect a debt.
People can do same thing like what banks do. They can create a contract of agreement between the lender and the borrower just to make sure that everything will be okay until the end. A normal lender has its own power even if he is just an ordinary person and not a bank. There are many things to think of, people borrows money when they really are in need. When loaning with bitcoin, I think it is better if you would loan the amount in USD but in the form of bitcoin for if the value increases, the amount of money borrowed will also increase.
Contract is the most value, however, for loans with collateral, I think it's hard to measure how much is the value of collateral if the collateral is a property, as anytime bitcoin will pump or dump, that would give the borrower an option whether to pay the loan or not and just let the collateral be foreclose.