Keep in mind that the allies need to see or hear the character so it can't revive characters that are at 0 HP.
DM's free to rule how he likes, of course. Both because the optimal use in 5e is often to bring PCs up from 0, and because unconsciousness doesn't include actual deafness, I'd tend to rule that it'd work fine.
It is good, but the Battle Master gets 3 maneuvers and the Eldritch Knight gets spells. Those are major benefits.
It's hard to stack up anything favorably next to spells. As you pointed out, a single spell, Healing Word, under one interpretation, is better.
Maybe I am looking at it from more of an identity viewpoint. The Battle Master and Eldritch Knight are well defined by their 3rd level abilities. Maybe in play Rallying Cry is that defining too.
Maybe not, but isn't it as defining as anything the Champion gets?
Are you saying "I haven't like high level play in prior iterations of D&D" or "I have tried high level play in 5th edition and do not like it despite how significantly different it is from high level play in prior iterations of D&D"?
It's always chicken-and-egg with high level, isn't it? Do people not like high level because it sucks? Does it suck because designers don't put effort into it because no one likes it anyway? Is it just that you generally try to get through low-level first, and groups break up before they get to high level? Where's the 'sweat spot' where it doesn't suck? (In 5e, there's a broad hint in experience progression: first few levels are lightning fast, then much slower for the rest of single digits, then speeds up again - keeping you in that mid-level 'sweet spot' longer).
Thanks to Bounded Accuracy, high-level play isn't /that/ different from what it was on the treadmill, in a basic sense, avoiding the 'why bother rolling' pitfalls of skill ranks, BAB/THAC0/Attack-Matrixes. High level spell /slots/ are limited, too, but powerful spells, plenty of known spells, & total slots still leave casters with bewildering versatility, so the 'weird wizard show' could still happen. As always, it's up to the DM to keep things on an even keel, but this time the 5e DM is Empowered to actually do so.