It's never quite explained how her powers work. For all they know it either won't work or will backfire.
It's never explained because it's not a matter of a concept of "powers" that "work" to do anything. The explanation in-series is simply that Eu has SUCH a RIDICULOUS amount of magical power that it tends to run rampant and out of control in that chaotic way magic tends to have when she loses the slightest bit of control/reign over it. This includes not only getting emotional and/or excited, but simply the act of speaking, of releasing words which, as per rules of most classic magic systems are an important element of magic itself, those words will contain the vast force of her magic and cause effects to reality simply by uttering them. She chooses her self-imposed muteness so that she doesn't accidentally cause things to happen. TL:DR, it's not a matter of being able to willingly warp reality the way she wishes.