Pretty sure Boadicea got aced because she was a woman just as much as Cleopatra got snaked because she was one. The difference being, of course, that we know a bit more about the latter than the former.
We also have no way of knowing if the story about the daughters, the flogging and the rest is true, since there are only three accounts about the whole ordeal and only of them mentions it. Either way, when Tacitus does mention it those acts aren't celebrated but rather denounced, and they're not even described as unlawful 'punishment' (so something that would be seen as acceptable punishment) but rather a consequence of Boadicea's husband's death and the chaos it caused.
Also, i'll admit i got used to topics derailing towards sexism and/or racism around here, but i gotta say it's the first time i see one ending up in roman history.