Hell if I know.

: Well they put out stuff saying it appears (to UNSC scientists) to be photons
: contained in some kind of field or something...

>.>

I'm hoping that the above is analogous to a tribe of neanderthals calling glass 'sharp ice that doesn't melt'.

: and the last I knew (during the lead-up to Halo 4) the energy sword is still
: plasma in a magnetic field, and since that's what seems to make the most
: sense for something like "hard light" to be and they never
: described it that way, I'm going going to say it's unrealistic.

Personally, I think of hard-light as something akin to Jackal Energy Shields, with several million more years of R&D.

: Weren't there large objects and structures and even whole buildings made of
: it in the books (the new books)?

There were ships that were mostly generated out of hardlight, and the exteriors and interiors of buildings were shaped/decorated with hardlight. In my mind, anything that's described as "Hard Light" ought to require a projector.