Misandra has the chance to deal a lot of damage with her special skill if all her additional 3 minor damage hits activate. On average, it is more likely that at least one will activate, sometimes two might, and rarely all three. Being fast somewhat mitigates this problem for her. Even if 1 hit activates though it can be helpful for at least the mana gain.
On the other hand, her damage is pretty poor if none of them activate. I don’t have her but I’ve fought her on raids where she activates her special 5 times, with none of the extra hits, and all my heroes are still standing. But then there are other raids where she destroys one of my heroes with one activation of her skill.
I recommend to take a look at this topic by Gryphonknight on Misandra’s Chain Strike, and then decide from there whether you want to ascend her or not:
On average, she will deal more damage than Richard though; just once in a while she’ll deal less damage than he would.
With 11 scopes and Frostmarch around the corner, she’s a decent choice to ascend imo.