There has been a lot of scare mongering lately that any aircraft that has not flown for a year is scrap. My experience is that even after 5 years there is generally very little wrong.
and then you tell people to run the engine on condition... I hope you fly a twin.
Obviously it depends on the storage.
I have a workshop at home, with loads of steel tools. I don't expect these to be rusty after 5 years, and they won't be.
But if I put them in the garden shed, which is the equivalent of a plane in an essentially unheated hangar, I would expect them to be rusty within a year or two, and they definitely will be.
Engines left around like that will be rusty inside. Not everywhere, but in areas where the oil has run off. About 10 years ago, a well known European a/c mfg stored a load of IO540 engines for 1-2 years past the limit date, before installing them. Most of the owners later found heavy corrosion. One I know personally nearly got a seized engine; I saw the photos of the cylinders. But it didn't happen right away; it took some months. I was one of the lucky ones; I got it only below the oil control rings, more or less, and in the SB569A job it all honed out OK.
It can happen below the top ring so just poking a borescope in there may not reveal it unless one looks carefully, turning the engine over.
Camshafts are prime candidates for corrosion too. And when they start wearing, they go fast. Remember that IOW 4xfatal PA28 crash? Their valve lift was 40% down, due to a knackered camshaft. Other mistakes were made but being well below book power didn't help. And you can't inspect the cams (on those engines) with a borescope.
Car engines cannot be compared directly. They have less outside air getting inside them. They have a long exhaust system, and a decent air filter.
The reason some people think this is OK is because most GA participants fly so little that it is usually somebody else, down the line of ownership, who picks up the tab for a knackered engine.