Like Sorrelhorse said, it's not a harsh bit at all. Due to the length of the shanks and where the mouth is located on them, it's got about the same pressure ratio as a regular snaffle. The only difference is that this bit will also apply pressure to the chin and the poll, giving you a bit of "lift and tuck" on a properly trained horse.
If she's flinging her head, my first stop would be the dentist to make sure her teeth aren't bothering her. After that, I would assume it is likely a training issue. Some horses who are ridden on short reins or who are cued too harshly come to hate mouth pressure...so they fling their head to alleviate it.
I see no reason why she wouldn't ride just as well in a regular old snaffle....though if it was me, I'd probably put her in something with a more solid mouth like a myler type and see if she'd start carrying the bit herself. That would help with the head-flinging.