Coaxial lines lend themselves well to the wire-bound transmission of high-frequency signals. The have no lower frequency limit and thus are able to carry DC, as well. Moreover, the propagation velocity is constant across the whole useable frequency range as long as the cable is kept in the transversal electro-magnetic (TEM) mode.
A disadvantage is the additional attenuation in the dielectric between outer and inner conductors. Compared to a WR-90 waveguide with 10.8 dB/100m at 10 GHz, a 1/2” Flexwell cable can merely offer 40.6 dB/100m. On the other hand, the waveguide is solid pipe and it is not easily bent which is no issue at all for a coax cable.