Are you actually saying that they are gonna get owned by the tribe that uses modern guns?
Its most likely a matter of skill
A master of the bow can beat a novice of the gun
True, in war anything goes. And the element of surprise is probably the most important thing. Ambushes and Guerilla wars FTW.... 
We could applicate that now into Altaux example, like, this Obeah has given out stuff that is basically water. Still, it could work.
But let's assume this guy is really into this and wants to do his job really. But then, the plants from which he usually draws his potion stopped growing it - unnoticed. And still it would work - *if* this is done in a region were residents have a strong belief in it. fascinating beyond grip that is.
Yes, this can work indeed, there have been numerous reports about patients with chronic pain problems, that were taking strong painkiller medication on a Vicodin-like level (kinda like Dr House, but true stories), and then their doctor would tell them that there's a new improved painkiller, that would have the same and even better effectiveness, so they could replace their meds with just a compressed sugar pill, and the patients would show the same response as when they were on the real meds.
This is actually the most effective use of Placebos, and it works more than 60% of the time.
"Mr Anderson, welcome back....we missed you...."
