It's not wise to go "omg broken! Perma ban!" Without the full check on her performance when she hits live server.
Maybe wait, see how she performs, AND THEN complain?
Like seriously, there's a saying "never judge a book by its cover".
I don't think she will be as problematic as Akali or a Zoe or even Pyke and Kai'sa. Her auto attacks is slower than Jhin. I mean the guy does some ballet dance moves while shooting his gun while Senna be like "damn it this gun is stuck! Hold on a minute so I can fix it before I shoot you!"
I Can imagine a skin for Senna, Garage Mechanic Senna where her gun is a rusty 100 year old engine.