Older road freewheels vs new mountain bike freewheels?
I don't think old freewheels are "better" by default. While it's true that most of today's thread-on freewheels are junk, that doesn't promote the junk of yesteryear to "desirable" status. However, perhaps the median quality level was higher back in the day.

In the days before freehubs, freewheels, like most things, ranged from the downright awful to the excellent. Regina made brilliant freewheels, as did TDC and several others, but they command stupid prices on Ebay these days. And even if you can find a nice NOS example, it will probably need a rebuild to run optimally.

Obviously there was a much bigger market for freewheels 30 years ago, with a higher profit margin, so manufacturers could afford to construct them well. These days it's less economically viable even to invest in quality tooling, let alone top-notch materials. In addition, today's manufacturing trends have shifted further towards light weight, at the expense of durability. Our disposable culture doesn't consider this a great loss, I suppose.

Perhaps it's fair to say that the old freewheels were built to last, but the vast majority of them, even in great condition, would be considered "a bit rough" by today's standards, especially when compared to modern freehub technology. Sadly there aren't any really high-end freewheels on the market today for a fair comparison...

1970s Regina = Ferrari

1950s TDC = Jaguar

1950s Atom = Peugeot

2010s IRO = Peugeot

2010s noname = junker