But all the data is still there, unencrypted. And I would not want to place my bets that there is no mechanism for going around the lock (UART, JTAG, etc.). You might also have sensitive data on the uSD card.
The only way to be sure no one can get to your (or to any third party data you might have) is to use full disk encryption. Then the stolen/lost device is basically just holding a lot of random data.