Actually not only its passive not unique leads to BC is OP problem.
One single BC always gives some AD caster a too big advantage against others.
Orginally, items with on hit stacking effects makes player to go attack speed and give up damage power, thats the way they trade off.
Current build one BC already gives champs skill with multiple hits to stack the effects EVEN with one single spell. As everyone mentioned, tarlon's W, pantheon's E, both guarantee a full stack combo to give then almost ture damage output on any physical damage output they do afterwards during early stage.
Wukong + MF with each one a single BC already at early game means a must kill under their simple ultimate combo.
I think riot should think twice about the idea of ad spells triggering bc passive. It no more make effect stacking items requires attack speed but simply ad building... (as long as they hv multiple hits spell) even make it a unquie passive wont solve the issue...
Word from top mained player.