you cannot competitively test champions on the pbe.
come on man
population is too low
matchmaking is all over the place because no one is actually placed correctly against a sizeable sample group.
like lol the only time the pbe will stop something going through is if it's lucian's pbe ultimate with an AP build, something actually demonstrably broken in all hands.
senna as she is right now is fine lol, still waiting for people to actually get good at her though, until that happens the champion barely exists and is most certainly not worth using a ban on.
also, as is the case with bottom lane releases, the other half of the lane needs to learn how to play with the champion.
adc's currently dunno how to play with senna, and the same for the reverse for senna players.
even better are the senna adc's which look set to be disastrous but worse things have certainly happened on the rift.