I've bought lots of pre-owned vehicles that I have absolutely no idea how it was driven before I laid my hands on it. To date, not one has developed engine problems that can possibly be contributed to improper break-in (excessive oil consumption, power loss due to low compression, etc.). Does that mean I don't believe in breaking in a vehicle? Hardly. It's a calculated risk, when taken with judiciousness, reaps rewards in cost savings. But it is a risk nevertheless. When I do buy a brand new one, I do treat it differently than I do a well-used one. It's more than just the engine. It's every set of mating parts that wear against each other. Whether it is lubricated by oil, grease, or nothing (like brake disc against pads), they all have to wear into each other. Regardless of materials you choose or the machining tolerances and surface roughness you specs (I spec those at work regularly), the manufacturing processes leave the parts with residual stress that will naturally want to relieve themselves out as the parts go through heat cycles and mating parts move.
Mfrs specify different break-in procedures. Some I admit are kinda overly simplistic and strict. But I understand they have to write it that way for folks with a wide range of mechanical aptitude, include those who have absolutely none. So they speak in terms that everybody understands. If it is too overly restrictive, well... at least it's a safer bet than being too loose.
The way I look at break-in on anything mechanical boils down to LOAD and TEMPERATURE. Excessive amount of either is never good for any moving mating parts in a motorized vehicles, regardless of age.
Lubricated mating parts normally rides on a thin film of oil (or grease) and never actually touch each other... well, not too much anyway. Every bearing surface, even polished mirror-like surfaces, has a surface roughness spec called out on the mfg drawing. When absolutely green, there are always some inconsistencies in surface roughness, as well as dimensional tolerances. Thin film lubrication layer is designed to be much thicker than the surface roughness, so the mating parts never touch. Under load, and/or when the engine on cold start, that lubricating film can be at its thinnest. Load will also cause higher shear stress in the oil film, which creates heat. Too much of that heat can cause localized hot spots, which in turn thins the viscosity of the oil right at that hot spot. That can cause localized viscosity breakdown and causes the thin film to collapse and the mating parts to touch, if only for a very short duration. This is why I take it easy before the engine warms up to operating temp and all the parts have thermally expanded to a steady state at that designed operating temp. It is also why I don't load the engine unless I need to. I do all the even after the vehicle is broken in. Before that, it is that much more important.
Regardless of what some may claim, if you constantly impose max load on the engine (say by redlining the RPM at all times), you will shorten its service life. That said, I don't buy a 125HP bike just to putt-putt around the gold course. I do drive it as I see fit, and I have no problem with it when the situation calls for it. The bet is, of course, that the amount of wear I cause in the time I own the vehicle does not exceed the total amount sustainable by design, before bad sh** happens.
There are other more nuanced consideration to other aspects of vehicle break-in, but I'm getting long-winded as it is.
'18 Ninja 1000, '18 Street Triple R, '18 KLX250S, '15 250XCF-W, '14 K1600GT, '05 GSXR600 (track)