[GeoffR]

I have plenty of books with auhor names like Sjöwall & Wahlöö, or series names like Bäckström, and I haven't come across any problems.

The only potential problems I can think of is if a character wasn't included in the font that the ereader uses to display the book's metadata, or if there wasn't a way to enter a character from the ereader's keyboard which could be a difficulty when using the search function.

Edit: Sorry, I didn't look carefully at your attachment, I was thinking about the metadata in the .opf file. But I think there would be even fewer potential problems with the html metadata, because ereaders will just ignore it anyway.)