Taking over territories with Assassins...

[P3D]

It is generally inefficient. Even Abysian assassins can lose to cavalry commanders, and the usual indie provinces has at least 3 commanders total.
Tribal provinces and Barbarians has two commanders. Horse Tribes only one. But there might be bodyguards.
The only time it is useful, is when you have access to assassins that can defeat commanders regularly (prophetized assassins, Abysia, Lanka, Patala), and your expansion troops would have trouble with elephant/heavy cavalry provinces.
Or you try to grab magic items from special indies.