UNUNs are in general deployed in transitions between a-symmetrical to a-symmetrical transmission line environments where only an impedance transformation function is desired.
This is the case where transmission line as well as load have an a-symmetrical nature. (i.e. no transition to symmetrical loads and thus no balancing function).
Typical usage is feeding verticals or long wire antennas referred to a good RF ground plane.
In order to eliminate or reduce common mode (sheath) currents, an additional line Isolator (See further on this web site) can be added further down the feedline.