Evaluating Blessings of the Blood god

Is Blessings of the Blood god worthwhile?

Heralds of Khorne, bloodthirsters, flesh hounds, and daemon princes of Khorne can all select (or already have built-in in the case of flesh hounds and Skulltaker) the Blessings of the Blood god upgrade.

For a modest sum of 5 points (double that in the case of daemon princes), the blessings provide the miniature with an invulnerable save against force weapons or aggressive psykers using their warp abilities to bring down Khorne's chosen ones. And to be fair: the invulnerable save is the best in the entire game! Who else can get a 2+ invulnerable save? There's not many. It's better than a storm shield on a loyalist terminator!

The draw back is that it can only be used in very specific circumstances: when a wound is caused by such weapons or powers. Hence it is somewhat unlikely that the miniature will find itself in such a situation. Sure, against opposing chaos sorcerers and loyalist librarians, it is going to be excellent. But knowing that the miniature has such a defence in the first instance may ensure that opposing miniatures suitably armed will preferentially avoid the Khorne miniature on the gaming board. Of course, the daemons player may be well advised to go after and directly hunt down such characters themselves (particularly flesh hounds who seem well suited to this chore), but often there may be better targets.

It is the other side of the ability: the resistance to psychic and sorcerous powers that may see more use. But again, knowing the existence of the blessing in advance may mean that opponents do not use such powers against the Khorne miniature.

Hence the blessings can be more of a deterrent than of actual utility in many games (perhaps with the exception of a squad of flesh hounds or a character paired against or intentionally going for a librarian, or equivalent). Therefore, for 5 points, it may well be worth while taking as an upgrade.

The final point that I want to make about this ability is that the way it is worded directly means it targets wound-causing sources. Against powers such as the chaos space marine Gift of Chaos power (the one that turns models in to chaos spawn) it has no use! It also has no use against daemon abilities such as warpfire -- they're not psychic abilities (page 73 of the codex).