Additionally, the unit body cars from the 80s up were designed with increasingly sophisticated finite element analysis tools. In the early 80s these were confined to critical parts because they were run on mainframe computers overnight, every night until the results were satisfactory.
As computing power got better into the 2000s, the entire unit body assembly was run on a desktop PC in 4 or 5 hours. Stiffer bodies was the result.
To make things more complicated, the biggest lump in the car is suspended on rubber mounts. A stiff body with a floppy engine still makes the car feel floppy.
Bad suspension design can have the same result. It can make a very good body feel like junk.
And then you have crash performance. Stiff parts combined with crushable parts that are less stiff than what they are attached to.
All these systems must be designed with respect to the others for the best result. Powerful computers for FEA and system modeling have helped a lot but they were not available in the 80s and 90s.
And finally a human needs to drive, evaluate and adjust all this because people buy cars, not computers.
One of GM’s biggest problems in the '80s was crappy assembly quality. Huge and uneven body panel gaps were commonly seen on GM cars of that era. In order to conceal poor assembly quality, they actually designed a Chevy dashboard that would make poor assembly quality less obvious.
But, this type of problem wasn’t limited to GM. My late aunt bought a an early '70s era Plymouth Fury, and the first time that I drove it, I noticed that there were small plastic pieces on either side of the steering wheel that hadn’t been “fully” installed. I didn’t want to crack the plastic, so I had to be very careful when applying pressure to properly seat those little panels, but I did manage to do it. When I showed my aunt what I had done, she thought that those parts were supposed to show a huge gap.
A couple of years later, my SIL bought a Plymouth Barracuda, which was one of the worst-assembled cars I ever saw. The paint looked like it had been applied with a broom, and I lost count of the number of interior panels that weren’t installed properly. The trunk lid was so badly-aligned that water leaked into the trunk.
