Does Nigerian Juju exist? Simplistically, yes, in the same way as Catholicism, or any other belief system exists.
Does it exist in a more meaningful sense? Yes, in that it affects the way that people behave and interact if they believe in it. Juju is a phenomenon that you would need to allow for and deal with if, for example, you employed a large number of people from a traditional Nigerian background. From time to time, there would be an incident in which their shared belief in Juju would be an important factor in managing your workforce.
Does it exist in the sense of actually working approximately as described and believed in by people from that culture? i.e. Is it actual magic that really functions in a practical way. Is there some way of projecting personal power to cause consequences for someone else, at a distance? I would say that there would need to be considerably more than wild stories, anecdotal evidence and hearsay before I would give that idea any credence.
However, if it did "work" as "actual magic" then would it be unique? Out of all the cultures of the world, are the Nigerians the only people to have stumbled across a system of ritual that has real effects? Given the long history of mankind, the broad range of cultures and religious beliefs, and the number of belief systems that include curses, hexes, and so on, it seems unlikely. The best I could say is that in the unlikely event that it is real magic, it is just one approach to operating real magic. For comparison, the English longbow and the shorter bow of certain native American tribes, and the complex bows used in the Olympics are all different practical approaches to using the same basic principles to achieve essentially the same thing
Can Juju be "explained away"? as the nocebo effect, hypnosis, psychosomatic symptoms, hysteria, and manipulation? I think very probably.
Does it work at a distance on someone who can have no knowledge that the "curse" has been issued? I'd say no, otherwise, a few Nigerian experts would by now rule the world. A quick Juju curse on the presidents of the USA, Russia, and China, the leaders of enemy armies... if it worked at a distance in a literal "it's really magic" sense, it would change the face of international politics.
But no, the reports all suggest that it works on people who are from one particular culture, or who have lived alongside it long enough to have absorbed some of its ideas. You have to believe in it, or half believe in it, for it to work on you.
However, the power of the word on the human mind is enormous, and applies across all or most belief systems and cultures.
Before the flight in which Buddy Holly and others died in 1959, Waylon Jennings said, jokingly, to his friend Buddy Holly, "I hope your plane crashes." After the plane crashed, Waylon Jennings struggled to cope with this for many years.
However, Buddy had said to him, "I hope your bus crashes," and the bus did not crash.
A completely rational mind, uncontaminated by the idea of the power of words, would have seen the contrast:
1) Buddy said he hoped the bus crashed but it didn't
2) Waylon said he hoped the plane crashed, and it did
...and seen that there was obviously no causal connection between the words and the event.