Disturbance observer (DOB), which is one of the key points of acceleration based motion control systems, guarantees robustness of a system by nominalizing real plant and suppressing external disturbances. Besides that, it can be used to estimate external forces/torques by identifying system uncertainties, and it is called as a reaction torque/force observer (RTOB/RFOB) in the literature. RTOB/RFOB has several superiorities over force sensors, and therefore, it has been widely used, specifically in the motion control area, in the last two decades. The main disadvantage of a RTOB/RFOB is that it is affected significantly by the identification of system uncertainties. However, there is no a clear report on the design constraints of RTOB/RFOB based force control systems. This paper shows that not only performance but also robustness and stability of a robust force control system are affected significantly by the identification errors in the design of RTOB/RFOB. A new design criterion which improves stability and performance of RTOB/RFOB based force control systems is proposed. RTOB/RFOB and force sensor based force control systems are compared and simulation results are given to show the validity of the proposed method.