Chris--I have done it both ways. My experience tells me that there is no problem with the "hard" break-in.

Obviously, since I build my motors, I know what was done and have no one to blame but myself if it blows-up. That said, other than starting it, getting the timing set and
checking for leaks and bad noises, why not put it on the track or a chassis dyno for some hard runs?

It does seem to work.

Maybe if I was buying expensive, pro-built motors, I'd feel the gentle break-in is better.

My car used to be street legal for break-in purposes. Not any longer!

Cheers.