Stupid ad, but I've owned tons of GMs and never really had dud. 84 Camaro is over 320kms with nothing but maintenance and wear items, plus and exhaust and an alternator. Escalade is pushing 260K and is flawless.
Their 80's/90's vehicles were pretty bulletproof, though. The engines (particularly the 350) were especially reliable. but they were also simple. None of them had cylinder deactivation, or stop/start, or variable valve timing. The chassis were pretty ordinary, they weren't trying to do anything magical with suspension setup, or whatever. It was all tried, tested and true.
I think when the 2000's rolled around is when they started adding technology to their drivetrains and started getting higher failure rates. That's probably also around the time they started to lean put more content into their vehicles, so in order to still make money on vehicles with more content while holding the line on price, they needed to cut back in some areas that couldn't be seen.