One problem that occasionally comes up anecdotally is: we work in a place focused on our target group and avoid all the others.
I suggest an alternate strategy:
1. Let people group thinking drive your selection of a place to work
2. Once in that place, work on reaching everyone in the place, regardless of whether they are part of your target group or not. This handles tiny groups and also forces you to build a strategy capable or crossing cultural and linguistic boundaries.
3. Use people group thinking to monitor your progress and make sure no barriers are left uncrossed.
This also addresses the issue where people pick smaller, "manageable" peoples and leave larger ones (i.e. the Thai) widely underserved. Rather than think of the large group as a monolithic block, we break it down by place (Thai of Bangkok or Pattaya or Phuket, for example.)