The offset is, as mentioned, predetermined.
In the case above, SetLeftHandOveridePose() is called by the grips equip function.
The copy nodes are not exactly demanding - they are copying the location and/or rotation of one bone to another.
Ditto with the offset node.
All the animation graph nodes are using Fast Path methods too.
The method described is far easier then having to create a custom pose for each firearm and grip combination. Far more flexible too, since the position of the grip does not need to be fixed to one place.
As far being reliable, been using it for years.
We have had issues relating to the left arm, but they are fairly typical of using IK - not the grip system itself.
i.e. elbow pole vector & over extension of the arm.