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Everybody always focuses on the wrong thing. It's not hard driving you have to worry about. It's using cruise control during the first 1000 miles.
During break-in, the zinc in the oil is being essentially cooked onto surfaces, bonding with the steel to make the surfaces harder and more resistant to wear. But since the surfaces are still somewhat malleable when you first get the car, there's still some wear. You get continuous overlapping cycles of wear, harden, wear, harden, etc.
The overall goal is for the engine to wear into a configuration that will provide the most power and longevity, then harden that way. The best way to do that is to provide the engine with a range of loads. In particular, under different loads the pistons will push against the walls of the cylinder differently and wear the cylinders into a tapered shape, where they're slightly wider in the middle than on the ends.
However, if you drive the first 1000 miles all on the highway on cruise control, you're not varying the load. You're basically breaking in the engine at one low power setting. The cylinders harden in a more cylindrical shape with no bulge in the middle or tapers on the ends. The engine then never develops the power it should. In extreme cases, you end up with the rings not sealing properly at each end of the stroke, resulting in blow-by.
I wouldn't worry too much about somebody doing donuts in this car before I got it. I would worry about buying one with 5000 miles on it that was broken in on the highway.
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