Not sure off the top of my head how that plays with mixed-direction arrows,
If you have arrows M0 ← M1 → M2, then it is also equivalent to an ordinary chain.
If you have arrows M0 → M1 ← M2, then it becomes inequivalent, due to collider bias. Basically, if you condition on M1, then you introduce dependencies between M0 and M2 (and also everything upstream of M0 and M2).
(I actually suspect collider bias ends up mattering for certain types of abstractions, and I don’t think it has been investigated how in detail.)
If you have arrows M0 ← M1 → M2, then it is also equivalent to an ordinary chain.
If you have arrows M0 → M1 ← M2, then it becomes inequivalent, due to collider bias. Basically, if you condition on M1, then you introduce dependencies between M0 and M2 (and also everything upstream of M0 and M2).
(I actually suspect collider bias ends up mattering for certain types of abstractions, and I don’t think it has been investigated how in detail.)