VTK was not implemented with UTF-8 encoding in mind, so without doing any code review or testing, it is more reasonable to assume that things don’t work correctly with UTF-8 content.
Just a few examples of why things may break: File path manipulation methods (get filename from paths, etc.) may not work correctly for some UTF-8-encoded paths (as you may have ‘/’ character as part of a unicode character). XML files written by VTK don’t contain declaration, so readers can only guess what encoding is used. Filenames are often saved into file headers, therefore by using UTF-8 file names, you introduce UTF-8 content to files that did not have it before.