Frederickson acknowledges that facial recognition is somewhat inherently insecure, meaning a picture could dupe it. However, a database of deny entries is maintained to improve reliability.
What makes Appellancy so much more secure is that it, by default, includes a database entry of random faces to populate a deny entry. This causes Appellancy to deny nearly any face except yours. By enhancing this database and the facial recognition methods in the future it will become even more secure. If you find a face that matches when it shouldn't, simply scan it into the deny entry Appellancy has to cause Appellancy to learn from that mistake.
The tweak is currently compatible with most known lockscreen tweaks, including, but not limited to: ClassicLockScreen, Convergance, AndroidLock XT, and iCaughtU.
You can purchase Appellancy from the Cydia Store for $1.99.