I wonder to what extent this is just habituation to styles that do not do what they are doing.

If I might, modern people tend to find cursive difficult to read. This depends somewhat on culture (nastaliq is the default in Iran) but is a kind of generalized trend that holds for most modern developed countries (see gyousho and sousho almost disappearing in daily use in Japan outside of menus and signage and - increasingly rarely - formal letters). It's not as if, I think, that when these older forms were more common that people struggled to read them (at least, not anymore than individual handwriting typically causes problems).

People who grew up writing cursive also often struggle with older scribal hand (though less so than someone who did not grow up with cursive), say from 1500s-1700s. Again, I think it's unlikely that the writers of those hands were so constrained by medium and technology (or cultural norms) that they chose to write in a way that was deemed inaccessible. (One might, if not attenuated to it, say that sousho is akin to deliberate cultural obfuscation, but my experience suggests that you quickly learn to recognize the patterns in kuzushiji.)

In the case of CJK scripts, brush pens haven't changed. Fountain pens are perfectly adequate for cursive (though some nibs were developed that differ specifically to make them even more suitable). For nastaliq, as for naskh, a reed pen is fine for both. (Modern pencils, ballpoints, and typical modern Western-tipped FPs do struggle to give nastaliq the line variation needed for a legible result). For Western scripts, pens and their tipping simply hasn't changed much beyond a decrease in the flexibility of nibs in FPs. (Something which also varied historically - pens oriented at most shorthand styles always had hard tips, excluding those shorthand styles that incorporate line-width variation which was meant to be achieved with a flexible tip.)

So my thinking is this is mostly something that comes down to 'are you used to it' and shifts in this area have a lot mroe to do with culture than anything else.

There are of course two other matters.

First, how easy something is to learn - I think the only place this is a consideration is sousho of the scripts I've mentioned (even with nastaliq's hundreds of thousands of possible ligatures).

Second, are the people around you accustomed (culturally) to the hand you are writing in, and how hard is it for them to adapt if they are not. Broadly speaking, people are not accustomed to reading much cursive in general these days, let alone one that varies from the recent hands of the area. So generally if one is writing for coworkers say, one would do well to simply write in print or at most a semi-cursive style.

In that regard the more that something deviates from its print form, the harder it will be to read for them. This ultimately comes down to interpersonal consideration - if you're writing for yourself or people who are regularly reading/writing cursive, I don't think the author's changes will be a significant issue beyond a short acclimatization phase that does not extend far beyond the phase that would be needed to adapt to an individual's personal quirks in a hand that has had some recent sway in the local area (and those hands do differ by country/area, quite a lot). (As a tangent, some of these tricks of the author's are commonplace in specific historical European hands.)