There are three factors that have been used to describe the status of a people group: unevangelized, variations of "unreached", and unengaged.
Unengaged is the newest. It means that a people group has a team of people engaging it with a church planting strategy.
Even a team of 2 or 3 is enough to count for this status. To be "engaged" is, as Paul Eshleman has said from time to time, not "the end" but rather "the beginning."
What we don't talk about very much openly, I think, is the reality of disengagement.
Many workers who go to the field do not stay on the field. We hear anecdotal stories of "once upon a time" when "workers sent their belongings to the field in coffins" because they expected to die there. (Side note: they expected to die quickly, too, because diseases hadn't been conquered yet.)
Today, a lot of workers wash out after 2 years due to a variety of issues that make them think "Maybe this wasn't my calling, really, to begin with." And more return home after 4 to 6 years, that being about the term of commitment of many.
Real effectiveness often doesn't begin until about the 8 year mark, and many don't make it that far. Few indeed are the workers who get to the 10 or 20 year point. I say this not to brag: I've been in missions research for over 20 years. I've seen a lot of people come and go. I've seen a lot of ideas come and go. This "come and go" has done as much as anything to keep the missions world from progress, I think.
More to the point: if an unengaged people group has 2 workers today, and those workers are less than 2 years on the field, it is highly probable (sorry for the Eeyore rain cloud here) that in 2 years that group will be disengaged.
What's the lesson? Everything a church can do toward missionary retention and endurance is important: better disciple development, better selection processes, better training, better support on the field. It's very costly--in every sense--to send a worker only to have them return home in 2 years, especially from foreseeable and avoidable circumstances.