RCS is pretty bad in KSP... I often have horrible oscillation when using it with SAS or MechJeb. Plus, it's really hard to design a good RCS system onto a spacecraft because you can't designate nozzles to do only one thing or adjust their relative powers. Easiest way to get RCS working well with translation is to put linear ports inline with the CoM, which makes them useless for rotations (except for wasting fuel), but if you add more ports with longer moment arms for rotation, that screws up your linear setup because you can't usually get them exactly balanced around the CoM. So in almost all cases you have unwanted translations when rotating and unwanted rotations when translating when you try to have a RCS system that can do both. And in all cases you have nozzles that are wasting RCS fuel since they are all firing if they think they can apply even a little momentum in even vaguely the direction you're turning/translating towards. Plus, if you have really tiny probes, even the weakest RCS nozzles in precision mode will still spin it at a billion RPM at the slightest touch. (This makes the reaction wheel-less probe cores almost unusable, because anything you'd want to use them in to save weight, you can't, because RCS would be too twitchy to actually be able to control them, so you need to add reaction wheels, and then you may as well use the next heavier core with proportionally sized wheels built-in!)