^Like I said, I hope I'm wrong.
If you're going to be worried about anything in terms of Swift's casting, you should have questions about whether or not she can handle the acting
part of the role, although I think it is even too early to have even those kinds of concerns.
Anyone who actually knows anything about music knows that you can teach somebody how to act, but you can't teach them how to sing. You can teach them how to unlock a talent for singing, but that talent has to be there in the first place.
We already know Swift can sing; we don't know if she can adapt the way she typically sings in order to accomodate a song the style of On My Own, but, knowing what I know about musical talent (coming as I do from a family that is very vocally musically gifted), I think it is far more likely that she'll succeed in adapting her singing style to accomodate On My Own than it is that she'll fail in doing so.
I'd also like to point out that the likelihood that the PTB behind the Les Mis musical movie cast Swift in the role of Eponine 'sight unseen' is rather slim; she had to have impressed them with both her acting and singing ability moreso than the other actresses who were up for the role, or else they wouldn't have picked her.