They are pretty sound old cars if you keep them dry. HQ-WB lower balljoints, chassis cracks, unserviced carbies with auto chokes and in HQ-HJ the plharness plug on the firewall are probably the biggest reliability issues, although to be fair the quality of aftermarket lower balljoints is the main issue and also people lowering the front greatly increases the incidence of cracked chassis rails. The clutch mechs can give you grief too. All older cars get the normal problems though: radiator, water pump, alternator, rusty exhaust, welch plugs, starter motor etc.

To be perfectly fair, if you were to get a country car with no rust eg a HJ tonner, go through it with quality suspension and brake parts, fit a good radiator and recoed electrics and run a recoed 202 and trimatic or recoed Holden V8 and trimatic, it'll go for another 30 years. In modern day though you are putting your life at risk driving one of these everyday, not from yourself but from the amount of idiots on the road. I have no doubt the head on I had in my SR5 would have ben a lot worse for me if I'd been in a HQ-WB.