I haven't seen the new IG book, so I can't comment on that angle of it, but in general I do agree that this is a problem, especially as it applies to morale. In 4th Edition, morale was easily the least important part of the game, since everybody and their cousin had some way of making their units fearless or nearly so. With the advent of 5th Edition, I was pleased to see that all those fearless enemies out there might have to start paying for the fact that they could ignore a statistic on their sheet *and* that those folks who weren't fearless would be suffering slightly more in the CC phase when they were getting beat down. Now, with the introduction of Stubborn and (if Hymirl is right) the spread of that rule, we are coming full circle again until I will soon be able to write off the morale part of the game again.
As for the instant death thing, I find that Eternal Warrior or its proxies are so common as to make anything causing instant death worthless. Honestly, there are very, very few enemies out there worth instakilling that you actually *can* instakill. Let's see here:
Stuff Wyddr Thinks is Worth Instakilling:
Tyranid Monstrous Creatures
Stuff You Can Actually Instakill From List Above:
Wraithlords and Talos (sort of--you need and Force Weapon, a Strength bump, some kind of special rule, or some mix of the three)
Greater Daemons (which, of all things, should be the one thing you *can't* instakill)
This makes instakilling a completely worthless special rule for just about anybody, unless you happen to need to kill the odd Chaos Spawn, Nob, or Ogryn in a hurry.
Don't even get me started and the virtual worthlessness of pinning, either...