First, from Charles Van Engen, in Mission on the Way (introduction):
[Missiology] attempts to allow Scripture not only to provide the foundational motivations for mission, but also to question, shape, guide, and evaluate the missionary enterprise.
Second, from Rebecca Lewis (pg. 36, note u):
Missiology must be based on seeing what God seems to be doing and evaluating that in the light of scripture (copying the apostolic process in Acts 15).
I believe there is a very big difference in these two approaches which explains why people in disagreements (e.g. insider movements) seem to be talking past each another. In the first approach, the Bible is integral to mission. In the second approach, the Bible is mainly used for evaluation.