One of the big issues in prioritization, on a global scale, always boils down to how people in various streams of Christian theology "handle" people in "other" streams. To put it more baldly, if Protestant evangelicals are estimating which parts of the world are "reached" or "unreached," "engaged" or "unengaged," "priority" or "not priority" - how do they handle Catholics? (and vice-versa). This happens no matter the scale that we are looking at. My own Stage scale (elaborated on a bit in the previous posts) assumes "Christians of all traditions" - but some may say "Europe really isn't Stage 3 or 4 or 5" because "Catholics and Orthodox don't count."
I would suggest a different view, however, in which the Stage approach very much does matter: places that are at Stage 5 (e.g. "greater than 90% Christian of any tradition") require a vastly different approach than places that are at Stage 0 (e.g., "less than 0.1% Christian of any tradition").
In other words, reaching definitely non-Christians (Muslims, Hindus, Buddhists, Jews, Sikhs, Atheists, Agnostics, etc)--and especially large masses of non-Christians--requires a different strategic approach than reaching "nominal" Christians or people who perceive themselves to be Christian but in fact may not be so in practice.
I would really be happy if equal resources were targeting each of the stages. The problem right now isn't really so much one of prioritizing Stages over each other, but that the lower Stages (e.g., 0, 1, <2% Christian) get so little in the way of resources, while the upper Stages (which are easier to reach and tend to be in languages that have a lot of Christian resources) tend to get far more.
(See also "Good, Bad, Non, Anti Christians and missions")