While browsing some Google results I came across a document by some
particular State/Municipality building department advising DIY
homeowners that junction boxes are not permitted in attics.

Is this an NEC rule or a local idiosyncracy? In either case, why would
this be bad?

I've discovered two metal junction boxes in our attic with the Romex
simply fed through the knockouts with no clamps. I was planning to redo
them with clamps, but now I'm wondering whether it would be good to
undertake a more radical upgrade.

Perce