I have unresolved issues with Scribus's justification process. I see books printed with beautiful perfect right margins, but I have been unable to achieve that, and I get two unacceptable-to-me results instead: some lines get all scrunched together with the glyphs narrowed, and others have great big spaces or even spread the letters of words. I have seen this kind of result often in newspapers and magazines with narrow columns.
So I have settled for settings which leave the right margin a little bit raggedy looking, but don't distort the text so much.
I have indeed experimented with the settings, although I probably could put some more time into this.
The automatic hyphenation function in 1.4.6 - in "Extras", by which I can select a block of text in a text frame and hyphenate it to help with justification, usually helps quite a bit, but doesn't make it perfect (sometimes it even makes it worse), and sometimes I am unhappy with the hyphenation. I have played with the settings here, too.
I conclude that it may be possible that the justification engine needs more work in Scribus; but also that I myself might need to work more on understanding typography. It also appears to me that different results appear with different fonts (controlled experiment needed).
It would be interesting to know who is achieving perfectly justified right margins, and what their settings are, and what fonts they are using.