In my opinion, her Slayer form works great against the many 4's decks that have arisen: BKR/Jack, Dilate/Ruuca, Catira/Toro in particular. She can also support Lycans against return-based decks in her lv3 form (but she won't get the Rats buff). Spy form is... I guess good against bigs? Haven't been able to use it yet.
Francis used to be a soft counter against bigs and mids, but her effect only works on 5 and up, and she's a lv4—therefore, difficult to fit into things like a Doll Rush, Lycan Rush, or (to a lesser extent) Undeads. Rivarre fills the gap left by Francis.
That being said, she's a support unit, not a centerpiece. Lots of situational versatility—she feels somewhat Falkow-y in that respect. I try to fit in 2 copies when i'm making a Lawtia rush.