To be honest it's nothing to write home about (thank goodness), as it relies on said heroes to have three or less wounds left in order for it to work, to the point that she could probably just kill them using her normal attacks. It's useful if she fluffed her regular attacks and gives her a second chance to finish off her target, but it's not that powerful at all.
I think what bothers me is that while it is extremely niche, it is also extremely reliable when it is relevant.
Creating a couple of scenario's where she'll be doing far more damage than is normal for a foot hero.
For example, an sunblood only averages like 2-ish damage against a 4+ save. Killing blow alone does similar damage.
So she can probably (assuming she gets a similar or better melee-profile) viably kill targets with 5-ish wounds in a single round of combat. Whereas the sunblood will need around 3 rounds to kill the same target.
That's a rather big difference.
Similarly, minor footheroes like a sunblood normally aren't real threats against big scary monsters, even when the monsters are at low HP. But she can.
For example, that sunblood can still realisticly fail to kill archaon, even if he has only 1 wound left (25% survivalrate for archaon at 1 wound? 75% at two?, nearly 99% at 3 wounds?) . Whereas she stands a much much higher chance of killing archaon, without even taking into account her actual attacks.
She's definitly not OP, at least not based of this rule alone, it's far too niche and specific for that. And if this special rule is truly a threat opponents will just shoot her of the table turn one anyway. But it feels like a weird rule.
To be honest, it kinda feels like a special rule that belongs on a fancy behemoth, like how some of them can eat a single model at the end of combat. Feels out of place on a minor foot-hero. Then again, maybe she just turns out to be the first impressive foot hero in AoS and she comes at a 200+ pointcost or something, in which case it makes a lot more sense to have a special rule like this
Looking at this and also looking at the reviews of the Nurgle and Fyreslayer tomes it certainly seems as if GW is actively trying to tone down the 'broken-ness' of armies so that they can provide some good combos but nothing that dominates the meta. Currently of course this means those armies are going to get stomped by the armies that had crushing meta dominance at the end of 2nd Edition (Vampires, Lumineth e.t.c), but if GW tones down those armies as well, then it could work out to produce a much healthier game overall, where better tactics wins out over super-builds.
That'd be nice.