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.
How you can avoid Eu's magic by not hearing it (as demonstrated in episode 6 when the killer removed her ear drums), but the tree behind her died despite not being able to hear the words?
Although the method is not fully understood, plants do "hear" human speech. Not only has empirical and anecdotal evidence shown that plants grow better when people speak nicely to them, several experiments have shown that identical plants respond differently to different human music. Plants exposed to continuous classical music grow better than those exposed to "soft" rock while those exposed to "acid" rock or "heavy metal" DIED, under all other conditions being identical.
So, if every word Eu speaks causes her pain, but simply "happens", why doesn't she just say "I don't want to feel pain when I speak, anymore."? Or even simpler, "I will not feel pain"?
Despite the comedy factor, is there any real reason to classify Ayumu as a magical garment girl? It is revealed in Maelstrom's debut that Ayumu still has his manparts. Thus, he's a dude in a dress.