King Pariah wrote:Wouldn't another weakness to these guys be any weapon or psychic spell or whatever that removes from tabletop or from play instead of outright wounding?
Not really, since the wording never says "wounded". The first part of the rule talks about horrors "slain as casualties". but the whole unit part is "removed at once".
So I am 99% sure it doesn't matter how the models were removed from play, you get to split them. The 1 exception noted is if Instability removed "the whole unit", but I think that also means that models removed from Instability still get split so long as the whole unit didn't di from Instability
Personally, I would have liked it if any models removed from instability not get split at all, since it is supposed to represent them getting sucked back into the Warp.
I think this rule is written to be overly complex.