In what sense a neutron star-black hole binary is the holy grail for   testing gravity?
  Pulsars in binary systems have been very successful to test the validity of general relativity in the strong field regime. So far, such binaries include neutron star-white dwarf (NS-WD) and neutron star-neutron star (NS-NS) systems. It is commonly believed that a neutron star-black hole (NS-BH) binary will be much superior for this purpose. But in what sense is this true? Does it apply to all possible deviations? 
Manjari Bagchi, Diego F. Torres
astro-ph.SR gr-qc