When getting down to the nuts and bolts of a Seebeck (thermoelectric) junction, the efficiency is highest when the heat flow is minimized and the temperature difference is minimized.
You get no power when those things are minimized, and in fact you want them both as high as possible.
As I understand they mean that a thermoelectric generator is much more useful in small scales of size and weight, where it can't operate very efficiently.
It's primary application being for spacecraft since it can have a decent power output for a very long time, which is what's typically required from a long standing mission.
I don't understand this part. How does a thermoelectric generator become more useful with less efficiency?