First please note:
PZM (Pressure Zone Microphone) boundary mics are omnidirectional: They pick up in all directions equally well.
PCC (Phase Coherent Cardioid) boundary mics are unidirectional. They pick up sounds mainly in front of the mic.
The text below refers to PZMs, but it applies to all boundary mics, both PZM and PCC. Here are some advantages of boundary mics:
1. PZMs are useful when the microphones must not be seen (stage play with mics on a table or wall, televised concert)
2. Compared to a conventional microphone near a surface (a mic on a desk stand), the PZM does not have phase interference with the sound reflected off the surface. So it has a flatter frequency response with no comb filtering. Compared to a conventional microphone lying on a surface, the PZM has flatter high frequency response. Because the center of the mic diaphragm is about ½ inch above the surface with a conventional “pencil” mic, it has phase interference at high frequencies which roll off the high-frequency response.
3. Compared to a conventional omnidirectional or unidirectional mic, the PZM has no off-axis coloration. (This is less true for PCC mics, but they still have very little off-axis coloration due to the small diaphragm size).
4. Mounting the PZM mic capsule very near the surface increases its signal-to-noise ratio and sensitivity by 6 dB.
5. Mounting the PZM on a surface increases its directionality. The polar pattern of the omnidirectional PZM capsule changes to hemispherical on the surface. The polar pattern of the supercardioid PCC-130, PCC-170 or PCC-160 changes to half-supercardioid on the surface.
Less off-axis coloration
Improved S/N and sensitivity