I have a draft of a book in which the different chapters are in different .sla files. I copy the styles and master pages to all - I have learned to make a template doc with the styles and master pages which I open and do "save as" when I start a new chapter.
In the doc setup menu, I can set the starting page number for each chapter. This would work with any project divided into reasonable sections.
Then I export each chapter individually to pdf and use a pdf-editing tool to merge the pdfs into one. The resulting merged PDF is fine to print from.
My only complaint is that when I copy anything from one of these files into another (say I decide something belongs in a different chapter) it imports the same styles from the other doc (all the styles being the same) and re-names them Style-1_copy (etc.) and so, if I do much of this, pretty soon I have to edit the styles and get rid of all the duplicates. So I copy first to a text editor which strips the styles out and then recopy into Scribus, an extra step, but it saves the time and annoyance of editing the styles all the time.
Trying to merge all the files within Scribus is something I have had no luck with because anything over about 40 pages takes too much memory for my computer. There are plenty of weird buggy things in Scribus (but the primary functions all work fine and well enough that I don't complain much) but so far I have found my own workarounds for everything, one way or another (even if time-consuming), and I respect the fact that it is a work in progress, free, generous, and not perfect, and if there are a few things that are broken or don't work right, it makes sense to look for the most immediate workaround and just do it.