The above stuff mentioned is stuff you assume should be there. Cars use nylon bolts, LocTite, and many other methods to ensure screws don't loosen down the road. Almost all cars don't leak, even after 20+ years of being on the road. A generator's mounting contributes incredibly to how quiet is is. If proper shock mounts, deadening vibration of pipes, hoses, and wires, and insulation is used, an Onan can be very quiet. Without it, it can be obnoxiously loud and replace a Magic Fingers bed.
Installation and design means a lot. Materials, fit, and finish can be the same, but there is a big difference between having each model wired the same, as opposed to wiring being slapdash and wound where the installer felt like it.
Look at RV roofs for example. 20-30 years ago, most were made of metal, and really didn't leak. Now we have rubber roofs that tear when a twig grazes them, causing leaks that can render an entire rig worthless other than scrap.