Typically, diesels are built with maintenance in mind (at least for trucks, no idea what you'd find in a diesel car). The cylinders are sleaved and meant to be replaced every couple 100k miles or so, so the cylinders themselves also last a long time (the sleaves take the brunt of the wear). I think theyre typically less complex than a standard gasoline engine. E.g. they have no spark plugs (and thus no distributor or ignition system), instead glow plugs to start the engine. Possibly more complex in other areas, such as emissions control.
I'm no expert on engines, so the the above may not be entirely accurate or current. Most of my knowledge comes from working for a semi truck/diesel engine manufacturer as an IT intern close to two decades ago and touring factories.
Nothing to my knowledge needs an overhaul yet, nearing 400k miles, and it's just been maintenance and consumables (except the turbo started leaking at 300k miles, which was replaced)
I'm no expert on engines, so the the above may not be entirely accurate or current. Most of my knowledge comes from working for a semi truck/diesel engine manufacturer as an IT intern close to two decades ago and touring factories.