I think you would never know you had a problem, they are just sitting there appearing just fine. You can cycle the handle when you need to and life is good. The problem is when they need to trip, they don't, and you end up with a fire.I've worked on a lot of Federal Pacific panelboards and they did the job. Of the ones I replaced, most were due to problems getting new breakers. While I heard from others in my local about occasional problems, I never personally encountered any.
Also, Federal Pacific made a lot of panelboards that weren't this Stab-Lock style and used a more universal bus and breaker design that probably never really had any issues. I come across those all the time around here and while those are 30+ years old now, you can still get new circuit breakers for them made by one of the big companies that are still around.