Merida was willing to kill her mother
There is one significant moment that I remember from the film that was probably the film's lowest point...the way the witch warned Merida of the possible consequences behind giving the spell to her mother strongly suggested (in my mind) that it could be something that could kill her. And still Merida agreed to use the spell on her mom.
Any human being with any concept of right or wrong would start questioning whether using such a spell would be a good idea at this point. I don't think any of the other Disney Princesses (regardless of unhappy they were) would go as far as to kill a relative to get what they wanted. Even Cinderella and Snow White have never thought about physically harming or endangering their stepmothers no matter how awful they were.
This is what makes Merida, to me, the worst of the Disney Princesses, even worse than Ariel. Ariel may have made some stupid choices but I think she would've given it a second thought had she known that it would've endangered her father's life in the process.