While traditional feedback control systems enjoy relatively good sensitivity properties, energy maximising wave energy converter (WEC) control systems have particular characteristics which challenge the application of traditional feedback and robust control methods. In particular, the relationship between plant and controller is largely defined by the need to maximise power transfer, and the controller contains a feedforward component which is difficult to robustify. Typically, WEC control systems are based on linear model descriptions, but this belies the true nonlinearity of WEC hydrodynamics (particularly under controlled conditions) and the associated power take-off (PTO) system. This paper examines two popular WEC control structures and examines the sensitivity of these structures to parameter variations, both in terms of closed-loop transfer functions and power absorbed. Some recommendations are also given on which WEC parameters need to be modelled with high accuracy.