Yeah they can be used anywhere but its pointless to do it. Im not saying you cant but they are not in the game for that. Of course you can have fun with free duels but SSJ/Kid/Great is not in the game for PVP purposes. I did some ranked battles and some new players was trying to use their SSJ. SSJ wastes precious time while transforming and you need every second in PVP. They are asking why a PVE transformation is not op at PVP.
You can break out of stuns with the transformation skills