Just tested PDF Bookmarks in Scribus 1.4.5
1. Surprise: I marked a frame on page 5 and after this a frame on page 3.
At generating a PDF with bookmarks, page 5 comes in the index list before page 3
2. Surprise I marked a frame on the English layer at bookmark. I started PDF export with the layers General and German marked.
Despite the layer English it was not marked for output, the PDF had now 3 index entries, 2 from the layer German, one from the layer English.
In this way, the index is complete useless, special when writing multi language documents.