  We study surface zero-energy flat bands in three-dimensional chiral
superconductors with $p_z (p_x +ip_y)^{\nu}$-wave pairing symmetry ($\nu$ is a
nonzero integer), based on topological numbers and tunneling conductance. It is
shown that the surface flat bands are fragile against (i) the misorientation
angle and (ii) surface Rashba spin-orbit interaction. The fragility of (i) is
specific to chiral SCs, whereas that of (ii) happens for general odd-parity
SCs. We demonstrate that these flat band instabilities vanish or suppress a
zero-bias conductance peak in a normal/insulator/superconductor junction, which
behavior is clearly different from high-$T_c$ cuprates and noncentrosymmetric
superconductors. By calculating the angle resolved conductance, we also discuss
a topological surface state associated with the coexistence of line and point
nodes.
