As clearly described in the documentation:
" As explained below, the initial configurations of intermediate
replicas can be atomic coordinates interpolated in a linear fashion
between the first and last replicas. This is often adequate state for
simple transitions. For more complex transitions, it may lead to slow
convergence or even bad results if the minimum energy path (MEP, see
below) of states over the barrier cannot be correctly converged to from
such an initial configuration. In this case, you will want to generate
initial states for the intermediate replicas that are geometrically
closer to the MEP and read them in."
So, if your transitions are "highly complex" as you said, you should
provide intermediate configurations.