When the Scarlet Witch is transported to the DC Universe her magic becomes so potent that even Superman becomes worried about the threat she poses.
While the strength of Scarlet Witch magic in the Marvel Universe is well known it can reach unimaginable levels in the DC Universe making Wanda so powerful that not even Superman could stop her.
In the J–L-A Avengers crossover series by Kurt Busiek and George Pérez the Avengers are transported to the DC Universe where they are given the task of finding six precious items in order to save their universe from destruction by Krona. On the other hand the Justice League must stop the avengers from collecting the items. In J-L-A Avengers #2 Wanda is initially unable to perform her hexes as she needs to adjust to the new dynamics of magic in the DC Universe. However once she is able to tap into the force she finds that it makes her chaos magic significantly more potent than it is in the Marvel Universe. The magic is so strong that with one hex Wanda is able to knock down all combatants in the area of operations. Even Superman and Wonder Woman confess to being concerned about Wandas dangerously powerful magic.
While Superman concerns about Wanda magic are in the context of how it gives her and the Avengers an advantage in obtaining the six items he should also be concerned about how it will affect him personally. Indeed since at least Superman #171 by Jerry Siegel and Curt Swan his susceptibility to magic has been DC canon. In that issue Mr. Mxyzptlk curses Superman with a spell where every innocent remark he makes comes true. Superman admits to his vulnerability to magic and the problems its causing him under Mxyzptlk curse.
If Superman could not overcome the power of a Mxyzptlk prank what chance would he have against Wanda reality warping existence bending chaos magic Wanda manipulation of chaos magic has already developed to astounding levels in the Marvel Universe but with the power up her magic gets in the DC Universe Wanda could take advantage of Superman vulnerability to magic and use it to his disadvantage. There no doubt Wanda should be able to easily subdue Superman and kill him if necessary.
The only wrinkle in Wandas magic use in the DC Universe is the same weakness she experiences using it in the Marvel universe namely control. Wanda Maximoff has had issues control her powers though shes gotten better at it over the years and with training. But it is clear from the upgrade in the power of her magic in the DC Universe that she will have more difficulty controlling it. With the amount of magic that will likely be necessary for her to wield against Superman the danger lies in what will the aftereffects be. That is in defeating Superman will the Scarlet Witch also destroy the MarvelDC multiverse