Pertronix are fairly reliable devices, but they do go bad. When they go bad, the car stops running suddenly and without warning.
Points are fairly reliable devices, but they also go bad. When they go bad, they do it slowly and with lots of warning, and can often be fixed with a screwdriver and / or nail file.
The most unreliable component in the ignition system is the condenser, but it isn't due to the design of the piece. It's the crappy after market units being sold. If yours has lasted more than a month, then chances are it will continue working for years. Don't replace it until you have to!