Most cars out there have issues unbeknownst to their owners.
I'd venture to say ALL cars have issues unbeknownst to their owners.
People leave the oil in their cars the first 6k-10k miles with all the metal grindings in it from the engine breaking in. That is also one of the worse things you can do to the motor.
In every one of my previous new cars I have changed the oil at 1000 miles and have never seen metal in the drained oil. At each 5000 mile change (Mobil 1 in every car) I never found shavings. Then, later on as I moved up to 7000 mile changes, same results and the oil was still clear. So in MY experience over the last 30 years, this is not actually happening and that's in engines from Ford, Chevrolet, Volvo and Honda.
And I've NEVER taken any time whatsoever to "break in" any engine much less think about it. I have followed the manufacturer's guidelines in the owner's manual to keep it below redline or vary the RPMs as much as possible over the first X miles, otherwise I have never done anything else in any new car I've ever had. And all of them lasted the entire time I've owned them until sold.
Many of those cars out there have lost 10, 15% or more of the original power, and their owners have no clue because it is slowly gradual over time.
ALL cars lose power over time and that is not due to improper break in but rather to normal wear and tear. Pistons do wear against cylinders, rings do wear out, compression lowers, etc. That's how all mechanical devices work.
An owner would have to show me detailed records showing dyno results on day 1 and at set times over the life of the car after "proper break-in" to convince me that does anything to prevent a 10-15 percent loss in power over the life of the engine over and above normal use.
And I will note there are plenty of people I know who have all bought several new cars over the years and none of them (several are true gearheads) have seriously broken in any new engine and I am at a loss to point out any one of them that blew an engine or had major issues because of that.
I think you should be careful the first 500-1000 miles but my humble opinion is there is no such thing as "proper" break in.