I think you could say there is a general consensus among the world outside of Christianity that Christian missions is nothing more than a form of colonialism. The argument falls somewhere along the lines of 1) they are individuals with human rights and are happy as they are, and therefore 2) who are you to impose your culture on them?
I’m no scholar, and I can’t speak for any tribal people, but last week I was shown a short video from New Tribes called Awayo: Fear to Faith, which I believe does a fair job of illustrating the Moi worldview before and after receiving the gospel. At the very least, I will let Awayo tell me what is best for him over the opinion of a guy in a classroom.
(Parents: I’d encourage you to watch this first before showing it to your children.)
Check out a higher quality version at http://ntm.org/video/, or if you’d like to hear Awayo tell his story in his own language (with subtitles), go to http://vimeo.com/7087303.