I find that struct_ctrb_obsv has a tendency to return zeros a bit too often, for instance in the following simple example.

julia> using ControlSystems.DemoSystems: resonant

julia> sminreal(resonant()*resonant())
StateSpace{Continuous, Float64, Matrix{Float64}}
D = 
 0.0

Continuous-time state-space model

The code has not been touched for a very long time and seems to be written by @jcrist 6 years ago, do you have any recollection of what it's supposed to do?

This function is used by zpkdata(::StateSpace) which in turn is used in, e.g., pzmap.