In my view, with a rental car you can at least know they got regular maintenance. I know they're driven harder, but the fact that they're getting oil changes puts them above the average mystery car for me, even if they're not ideal.
My brother has told me similar stories, though I can't recall specifics at the moment. Most were similar to my cousin's stories, the theme being "as mechanics, we didn't do anything except make sure there was air in tires and wiper fluid."
I think the companies they worked for probably knew what they were doing too. Most cars made within the last 20 years can go a hell of a long way without an oil change and still function (which isn't me saying that "function" means the same thing as "work well long term"). Rental car companies know when to sell them before they stop functioning.
My wife and I had to exchange a rental because it was producing excessive exhaust, the alignment was so bad it pulled, and an engine service indicator was on.
Rentals are frequently driven hard and may not get basic routine service (that should happen more frequently due to being driven hard).