Forbidden Arcana does have some nice things in it so I'm curious why you view its rules as especially bad compared to other magic sourcebooks.
Forbidden Arcana has some great things, but they make up a puny amount of the material. You're not on the errata team, so you can't see the ten threads I've made on it's various chapters and how big of a shitshow it is.
To summarize...
The Mastery qualities are wildly unbalanced, and several don't actually work. A couple are superfluous. And it goes from laughably weak to incredibly broken. Some break lore.
The Blood Magic section has a lot of useless spells that are just normal spells but worse, a metamagic that doesn't make sense, and the whole Blood Crystal thing is really out of place and unneeded.
The Traditions are a fucking travesty. Some of them should just be Magical Groups, but the writers didn't know that was a thing. Others just give bonuses for no reason. There's the "Traditional" version of traditions that has no explanation as to how you qualify or if you have to purchase it. Several of the traditions are laughably bad, and only give penalties. One just gives a player toxic magic. Three are based on a separate, unrelated IP (the Swamp Thing mythos for the record). Drakes have their own insultingly overpowered tradition.
Tarot Summoning. Fucking Tarot Summoning. It's not only overly complex, it's utter shit, and costs 107 Karma to unlock fully, minimum.
The book includes a spell that doesn't make any sense, but makes food, which brings up a lot of questions and messes with the setting.
The entire "Focused Awakened" section is unusable because it doesn't give any rules for how you can actually BE one of those kind of Awakened.
The Harmonious Defense metamagic is totally overpowered.
The inclusion of a metamagic to just become an Insect or Toxic magician is awful and makes it seem like they are options available to normal characters.
The Spirit Leash mechanic is so punishing as to be totally unusable.
The book is full of options that makes your tradition mean less and less, both mechanically and in-story, which is a bad thing. I don't like that this book would let someone play a Olympian-Voodoo Magician summoning possession spirits of Greek monsters and also this other Christian spirit they just figured out how to do, plus these other two Hermetic elementals they got from taking Chain Breaker.
So much of this goddamn book is poorly written, rushed, not thought out, and the errata team has to rewrite most of the rules, and try and think of ways to "fix" what should just be removed, but can't be because word count and layout.
There's more, a lot more. If you'd like I can give you a Pastebin of everything I've analyzed about the book (as what I've written isn't using any kind of insider knowledge, so the NDA isn't an issue). It's a fair amount though.
Because they don't get to open the book and see "Oh, I am using Shedim summoning"? All they get to know is what they have been taught and not every mage in the world spent 4 years at MIT&T. If your mentor tells you "here, this ritual will help you summon grandma" you think you summon grandma. You are using a lot of meta knowledge to determine that it isn't worth it.
No, but you see a shadowy, horrifying spirit appear with an aura of fear that you are
not immune to that doesn't seem like your grandmother in any way, shape, or form. You've seen spirits before, after all, you have the skill to summon them and are at least Initiation Grade 1. Then when it enters her corpse, her body is wreathed in darkness or some other sign of possession, and she does nothing but follow your orders, and after a task or two, the shadowy stuff goes away and she collapses on the ground again. You try to talk to her, but she knows nothing about her past and basically acts nothing like your grandmother did. Can't even make her signature pie anymore when you command her too...
Though to be fair, they have learned it at this point. I just think this random country bumpkin would probably be horrified, feel sick at the mockery he made of his beloved family member, and swear off ever summoning one of those "things" ever again... Thus being a waste of the Master Shedim's time, since no summons = no Shedim escaping into the world.
Yes, if someone who is as ignorant as possible and has never learned about the massive historical event that involved Shedim trying to take over the world, but somehow has initiated and is found by a Master Shedim, they could be tricked as long as they ask no questions and believe everything they hear. A Shedim could certainly teach someone who is basically tabula-rasa when it comes to their understanding of magic and world history, grooming them from early in their Awakened life into a Shedim summoner.
You can't think of a use for animating corpses? Especially well armed and armored corpses? Or corpses of important people?
What? No. But you can do that with non-Shedim spirits. This is not something exclusive to Shedim. Any spirit capable of possession can do it, and it's not something so useful as to be worth the backlash that would follow anyone seeing you summon a Shedim. Also,
Shedim do not have the Realistic Form power. So that "possess the corpse of someone important" doesn't work, when they are obviously possessed (which is how it works, and avoiding being obvious is one of the features of Realistic Form).
So you are saying it is both too weak to bother with and too over powered to be allowed and is in need of a nerf.
Read what I write before making strawmans. I'm really not in the mood. I'm saying they are too weak to use as simple Summons, but using
Channeling specifically changes it dramatically into something quite powerful with an ability that needs to be errata'd. Two different uses that affect their power level. On their own, I think they are kind of crap. With Channeling, they break the game because they allow the player to access something that should never be accessible to players. Nothing I said is contradictory.