I've read and heard lots of horror stories for years about these circuit breakers and electrical panels.
One electricial even went so far as to say he delibrately tried to trip one by grounding the service wire (14 gauge romex) and the wire started melting with the breaker still not tripping!!!
All the data I've read about these things say they work fine "most" of the time and the only problems occur when you need them to 'trip' like during a short circuit. The failure rate at those times can be as high as 60% or more.
The Consumer Product Safety Commission has tested these breakers and found them defective and not able to pass their tests.
Failed electrical panel box
Federal Pacific Electric got these breakers UL (Underwriters Laboratory) approved but was later found to have rigged the certification tests during a class action lawsuit in New Jersey.
Bottom line: lots of home inspectors call these electrical panel boxes and breakers "fires waiting to happen". If you have any in your listings or homes your selling you should have a licensed and qualified electrician examine these breaker boxes to determine if they're safe.