for me overpowered is being too broken for your own universe's limitations. you win every battle without fail.
itachi is that kind of OP. he has hax. after hax. after hax. after hax. its frankly mary sue terroritory.
kakashi would be the same thing but kishi made him a pacifistic puss y who DOESNT warp everyones heads off in two seconds.
madara is strong in the physical sense, much like hashirmaa. they arent broken, they have just trained their abilities to the highest level.
overpowered in the conventional sense refers to beings like rikudo sennin and one above all. they arent broken, because they are the ones who created what made that character broken. they arent broken because they are necessary for the verse to exist as their creator god.