No, you will always get paid the interest you are owed. Poloniex and the other exchanges that support this implement margin calls, meaning that if a borrower suffers large losses they will be forced by the exchange to close their position and repay loans before they become in danger of defaulting. A default is technically possible if market conditions are so extreme that the built-in safeguards can't close out a position fast enough, but the chances of that happening are extremely low. In all the many months I've been lending, there has never been a single default.