Question on same faction/different army ruling

[LordPathos]

So we know that Tyranid Psychic powers cannot be used on GSC units. Nid FAQ states:
"In the rules described in this section we often refer to ‘Tyranids units’. This is shorthand for any Tyranids unit that also has the <Hive Fleet> keyword."
Does that mean this type of rule does not apply to other armies? Ex: Diabolic Strength from CSM, you target a Adeptus Hereticus model, it now has +2S and +1A. Does that model have to be from the same legion? If it followed the Nid example, it would have to be Adeptus Heretics and <LEGION>.
I'm assuming, because this is in the Nid FAQ and NOT the Chaos FAQ, CSM Pskyers can buff any Adeptus Hereticus unit? Would this apply to any ability, stratagem, etc that doesn't specifically require <LEGION>?
Thanks

[No One]

So, this is actually a terrible Errata, because it doesn't do what it says it does in the BRB FAQ: that's the one that says everyone gets everything except GSC. (Compare and contrast with the new Ynnari rule which does the same thing that GSC is supposed to do, by actually being written into the rules ).

[LordPathos]

What are you referring to? I haven't gone through the Ynnari stuff yet. Until I reread what you typed, I thought you meant how Craftworld units can benefit from Craftworld powers.

[No One]

Ynnari can't benefit from Craftworld/DE/Harley strats, powers etc, and it's very explicitly spelled out in the rule that they don't. Unlike GSC, which is basically just the same 'can't access' as everyone else, who can benefit from everything.

[LordPathos]

In terms of strats, is it the same? Like CSM strats dont affect DG units right? Ex: can I use CSM chaos familiar (pick an adeptus hereticus pskyer, replace a power with one from the Dark Hereticus discipline) on Morty?

[No One]

No, Ynnari (with new rules), GSC and Daemons are the only ones who can't share strats with applicable keywords. So using familiar would be fine if Morty has the keyword and you have a CSM det to gain access to the strat.

BRB FAQ said:

Q: Is it possible to use a Stratagem from one codex to target a unit
from another?
A: Yes, so long as the unit the Stratagem is being used on
has the appropriate keywords (note that the Stratagems
in Codex: Chaos Daemons and Codex: Tyranids are specific
exceptions to this, as discussed in the relevant FAQs
and Errata).
For example, you can use the Tide of Traitors Stratagem
from Codex: Chaos Space Marines on a unit of Cultists from
a Death Guard Detachment (from Codex: Death Guard) if
your army has both an Alpha Legion Detachment and a
Death Guard Detachment in a single Battle-forged army.
Also note that the only requirement to have access
to Stratagems is that you have a Detachment of the
appropriate Faction.