As far as the electro-priests are concerned, they're appallingly hard to use. On the one hand, they're meant for front line combat. Either to get into close combat, or to use their silly 12in shooting attack. On the other hand they cost as much as veterans and have less than adepta sororitas stats, with an even smaller threat bubble.
Since the book has no transports to speak of, the only way to -kinda- mitigate their flaws is to take big packs(upto 20)
Which could be great if there was any reliable way of handing out infiltrate/scout/outflank, but there isn't, which is dumb, there's not even a cult mech warlord trait for it, and also begs the question WHY DOES INFOSLAVE SKULL GIVE ACUTE SENSES THEN?
It could be said that they're meant to flood in behind the Kastelan robots, or perhaps rent an allied landraider crusader.
But now as you can see, I've just spent almost 1000pts just to formulate a gameplan to try and effectively use a unit that you probably didn't want to take in the first place. I'm trying to be optimistic about the unit, but really it's just very unwieldy and weak. Like a tin foil razor blade.
I believe that every unit has its purpose and situation, but they're just too damn fragile to perform. Scatterlasers shred them, and we all know how prevalent those are.
I suppose it's also worth mentioning that there's a formation, minimum 360pts and gives them crusader usr, and some other quirky rules that don't justify the investment, but that's just my perspective.