I run a rune priest in some of my lists. I keep the equipment pretty minimal
I mention the equipment for context, but this is mainly about how the rune priests are played in the game

In the lower points lists, I run him as he is. Living Lightning, Jaws of the World Wolf. Pretty solid, Living lightning gives some fire support on top of the long fangs. Jaws, if the opportunity comes up.

At higher points, I equip him with a jump pack. When there are points lying around, Chooser of the slain. I take that primarily to deny an infiltrate position. The chooser is usually the first thing to go when I tweak my lists. Storm Caller, Jaws. At higher points I have enough missiles in the army to deal with anything that would have been shot by living lightning anyways. Storm caller gives me that very useful 6" bubble. Occasionally, the unit in front might go to ground to get 4+ cover. Anyways, the jump pack lets me move him around better for the placement of that 6" bubble. Of course, I keep him attached to squads, but I can un-attach him, move by jump pack, and them attach him to another squad (one that has already moved as well) all in the movement phase. Also, he could simply footslog with the squad. In any case, the flexibility is usefulness. Finally, if the opportunity comes up (somewhere in the neighbourhood of nurgle daemon or necrons geting too close), I can cast Jaws. And with that jump pack, I can better position him to get the most out of the spell.

I'm not an eggs-in-one-uberpowerful-basket type. The rune priest supports the army fairly well, but is not so essential that if he got picked off early somehow, that my army is unhinged. I have no ridiculously kitted out units, but a very, very high model count for a power armoured army. The only "costly" things are Bjorn and Thunderwolves, and only in one "fun" list do I have both (this list is engineered to have all my sweetest models on the table). The rune priest supports the army with the storm caller bubble, the nullify spells bubble and the denied infiltrate position with Chooser. If something like Dante lowers all his stats or something, his usefulness isn't diminished. I don't rely on him (or any single unit, except maybe Thunderwolves) to do shocking damage, but rather the whole army. Probably comes from me starting the hobby with Tau.

Anyways, I'm just throwing ideas around, surrounding how my rune priest is played, with notes about my army just for context. The rune priest HQ is, I think, not so much about uber-killyness, but the judicious use of his abilities. I'd like to hear some ideas and success (or fail) stories.