Maybe she could train it later, but so far the actual powers Eri has shown are nowhere near as granular and easily used as people have been fearing.
-She instantly rewound her dad to seemingly nonexistence
-When Chisaki used it on the mouse, it seemed to instantly rewind to some tiny smears of organic matter
-On the quirk chart with the families, Eri's was represented with a goopy stain
-Chisaki seemingly knows what he's talking about regarding the science, so I don't see him overlooking the "well maybe it's not permanent, it just rewinds the Quirk to its birth state and then redevelops" argument when he insists it's permanent. I think the temporary bullets just weren't potent enough, and didn't get all of the target's genes/plus-alpha, allowing the body to regenerate them, while the finished version can instantly spread through the entire body and instantly rewind all of the genes/plus-alpha to base organic compounds before the body can react, in which case there's nothing to redevelop over time.
-I've seen people use her breaking up Fused Overhaul as proof she can choose exactly when on a timeline to rewind a person, but under Shonen Law™, a fused person is a new entity. Chisaki blew himself up as part of the fusion process with Nemoto and mixed them both together, and therefore I think what happened is she automatically rewound "Nemoto!Chisaki" to the point that "it" no longer existed: "it" was automatically reduced to "its constituent parts": Chisaki and Nemoto and some rocks I guess. Eri had no conscious control over it.
As a result, I don't think she (at yet, at least) has the ability to do something as fine and precise as fixing a dude with a hole in his chest, or a Quirkless individual: she'd at best just make them a toddler Mirio who's permanently quirkless, or a Nighteye with less wrinkles but still a missing arm and hole in his chest.
Now watch Horikoshi actually explain the rules of her Quirk in the coming weeks and it's totally different to what I'm saying.