As with everything else in life, there are always trade-offs that must be made. The trade-off with the COT devices is that ripple is needed on the output voltage, and on the feedback pin. This is required because there is basically a comparator in the controller, and that comparator needs some overdrive. Therefore it is mandatory to have some ripple voltage on the output, and the ripple voltage must be proportional to the inductor current. The easiest way to accomplish this is to add a resistor in series with the output capacitor. This will add the ripple that is needed for the regulation, but it is also going to have the undesirable effect of having some quantity of ripple on the output voltage. Plus the ripple will have some variation. The amount of variation will also affect, to some degree, the regulation response or the DC response as well.