RE: handheld comm devices
You think, it's weird, because the Eldar could communicate telepathically or via comm-implants. (Btw, in reality, we are just a few years, maybe one or two decades, away from decent bionics.)
But I just stumbled upon the perfect excuse: Maybe Eldar Guardians simply aren't equipped with comm-implants and the Shadow in the Warp prevents telepathic communication.
Well, yes, they DID fuck things up, really bad.
But on the other hand, the Eldar with their knowledge and the Tau being the only race that seems to actively develop and research new technology, could be the only ones able to stop the Nids in the long run.
The Imps use their usual tactic and try to drown their enemies in all the blood and the orks.. haha, the orks :D
Theoretically the Infinite Circuit would pick up breaches like that at least as disruptions in it's branches.
Unless our unfortunate friends have been cut off by a Nyd breach of the IC.
It's war. Any number of unforseen things could by why.
Handheld radios are kind of anachronistic, but Eldar gear seems to vary wildly by the needs of the author of the novels anyway, so whatever.