A lot of folks think that Knuth-Plass is the optimal solution for good looking text, for all text. It really only considers Latin, and even then has restrictions.

Some fonts have the space character in the kerning table, and possibly (although super rare) in the "ligature" table (gsub). (https://www.sansbullshitsans.com/ is an example of a font with a space in gsub, "paradigm shift" maps to a single glyph).

If you want to go really deep on how complex some scripts are take a look at: https://r12a.github.io/scripts/arabic/arb.html#shaping

By "shaping across space" I mean that the shapes of the words might be very different depending on whether there's a space or whether they're on second lines. That basically doesn't happen in Latin script (though one could possibly imagine a very fancy cursive or a stunt). So I'll give an example in Nastaliq. One of the example images[1] on Wikipedia is "خط نستعلیق" (meaning "Nastaliq script") where the "خط" is tucked under the other word when they're on the same line. There's a space between them, so it's a valid line break. If they were on two separate lines, the total width would be a lot wider because of that shaping behavior.

This is not conceptually a difficult problem, you can just shape all possible substrings between candidate line breaks to get their widths, at which point Knuth-Plass will give the optimal line breaks (relative to your objective function). But given that shaping is expensive, you really want to avoid that in the 99.99% or so cases where the width of the word isn't altered by other words beyond space boundaries. That's what the "safe to break" logic is about - letting you know when you can make that assumption, as opposed to needing to reshape to get the precise metrics.

[1]: https://en.wikipedia.org/wiki/Nastaliq#/media/File:Khatt-e_N...