why is it that either mana barrier mucks with all sustained spells
Sustained spells (and all other active magic like vampires, magician using astral perception, spirits, activated foci, wards, mana barriers etc etc) all have a tangible astral form on the astral plane. If you try to move an astral form through another astral form then you risk causing astral intersection which might lead to disruption.
...but only block certain spells actively being cast through them?
First of all:
- Spells, not just mana barrier spells, can only be cast on one plane. Either the physical plane. Or the astral plane. You can never cast spells on both planes at the same time.
- Mana barrier spells don't "block" spells from being cast through them, they act as "Boosted Defense" against spells being cast through them (on the plane they were cast).
A magician on the physical plane using Boosted Defense (counterspelling) will not add defensive dice against astral spells (same as a magician casting a mana barrier on the physical plane will not add defensive dice against astral spells).
A projecting magician using Boosted Defense will not add defensive dice against spells cast on the physical plane (same as a magician casting a mana barrier on the astral plane will not add defensive dice against spells being cast on the physical plane).
Wards and medicine lodges etc act on both the physical plane and the astral plane at the same time (they are considered
dual natured active on both the physical and the astral plane at the same time) so they add defensive dice against spells being cast on both planes.
Edit: They are still mana barriers though, not physical barriers. Even if they add defensive dice against spell being cast on both the astral and the physical plane they still only have an actual astral form. They don't prevent physical objects from passing through them.
Edit2: (except the circle of protection which actually act as a physical barrier, an astral mana barrier
and a physical mana barrier at the same time...)
I don't think anyone is saying that only some spells are inhibited?
Unlike all other mana barriers (rituals, lodges etc) which add defensive dice against spells being cast on both physical plane and astral plane, mana barriers
spells can either only be Astral of Physical. An astral mana barrier will add defensive dice against astral spells.
SR6 p. 161 Mana Barriers
Astral mana barriers are resistant to astral spells