The 39s used to be a popular law enforcement gun. I repaired many of them when they were popular. You hit on one of the three problems. Seems like a hit and miss hardening process on the extractors. Some were too hard and broke, others were too soft and wore out.
Another problem was broken firing pins. Same issue. Some were too hard and got brittle.
The most serious problems were with the barrels. They frequently cracked just in front of the chamber. I replaced a bunch of them.
The mod 59, a double stack 9mm brother, had the same problems. Of course, there were thousands of guns that worked perfect and you never hear about them.
I liked them for the same reasons. Nice feel, good sights, near-perfect function. I wouldn't be afraid to buy one. Closely inspect the barrel and extractor. Don't shoot hot loads (factory included) or dry fire and you'll probably be fine.