From what I've seen, her E is a powerful engage, especially against immobile squishy champions (Ashe, Jhin, Sona and some more).
Even if her W isn't that useful in most of those match-ups, she can still use it against most of the junglers (Lee, Gragas, Nidalee).
Also, I think she tends to be snowbally... Her Q and her E have so much power in general, that she can literally be oppressive in the early game. If she manages to do that, her opponents will most probably miss Gold/Exp and will get their level 6 power spike delayed. While her Ultimate may not have an amazing impact when engaging, she can make it happen before the opponents get theirs.
If your ADC picks an all-in hyper-aggressive champion, then it is a win-win situation.
Overall, I find her success based a lot on what match-up she has, so a bit more of a niche. But she is indeed viable and efficient.