Another disadvantage is that private loans have a fixed repayment schedule. After a certain point, like 4 years after you take them, you have to begin repayment. A private bank has every incentive to take you to collections if, 4 years after you start med school, you aren't ready to start paying up. If you have to take an extra year to finish school, or change your mind and want to go to a different school, the bank is going to demand their money.
8.5% may seem high, but consider this : the best fixed interest rate 30 year mortgages you can get are a touch over 5% interest. Those mortgages have collateral : the house. So, no matter what happens, the bank can seize the house and get a good chunk of their money back.
For a student loan, there's no collateral and considerably more risk. If you default, there's nothing the bank can really seize. So, however the lending agreement is set up, it's basically guaranteed that the bank is going to charge you lots of money in interest above what a mortgage would be. The "variable interest" is one such scam : interest rates are extremely low right now. They will get much higher once the economy recovers and the Fed raises the rates back to normal. Another thing a private lender does is that the interest compounds quarterly or monthly, not after graduation like for PLUS loans.