They are cheap and useful, but there are the following problems
for sensorizing the tiny things. The size of the board
is relatively large. In addition, the final deliverables usually
become larger, because the base board requires various
additional functions such as a charging circuit, clock, storage.
Moreover, each additional function has a connector.
As a result, it is actually difficult to develop a "smart" sensorized
things. Another problem is a software for controlling
the deliverable from a smartphone. It is not easy to develop
a sophisticated mobile application for each deliverable respectively.