I mean, the dictionary definition of にくい as a suffix is
動詞の連用形に付いて,…するのがむずかしい,なかなか…できないなどの意を表す。
(emphasis added by me)
Of course, you can still have nuance with them, but it’s more like にくい is a subset of のがむずかしい if anything. Meaning のがむずかしい should be acceptable for にくい. Though perhaps not always the other way around.
EDIT: Sorry if I’m missing something referred to in your link. I can’t access it on this network.
I’ve honestly just thought of it through English equivalence, which remarkably - at least in this case - seems to hold up.
I’ll use one of the examples in that article. 日本語は話しにくい。“Japanese is difficult to speak.” That doesn’t sound right to me even in English. Because it’s like… Huh? There are certainly a lot of people in Japan who prove otherwise! Doesn’t make sense to give that kind of intrinsic attribute to the language.
On the other hand, ~難しい (“~is difficult”) to me just has a different nuance. Continuing again with the direct English, “Speaking Japanese is difficult” I think has some implied meaning like “for me” or “for learners” or “at this moment” or whatever. It just makes more intuitive sense.
A relevant example and conversation point I’ve had with native speakers is on Ryunosuke Akutagawa’s short story In A Grove. この本は読みにくい. This is a difficult to read book. To me it’s a more universally agreed upon characteristic of the book - due to say the grammar and vocabulary of a book written before the war. Agreed upon even by native speakers. Whereas “Reading this book is difficult” just has a different meaning… like maybe it’s because I haven’t studied enough, or the lighting is dim, whatever.
Maybe to articulate this better, at least in the case of “this book is difficult to read” vs. “reading this book is difficult” (読みにくい or 読むのが難しい) we change the “target” of the difficulty. In the former case, “difficult to read” is an adjective applied directly to the book itself whereas in the latter case, “~is difficult” is applied to the act of reading it.
I think that holds true for another one of that article’s example’s of “crab is difficult to eat” or “eating (all this) crab is difficult.” The former is an inherent characteristic of the crab, and the adjective describes the the crab itself. Whereas the latter case applies to the nominalized version of the verb - the act of eating it, due to some current circumstance.
Not sure if that’s a great explanation but it’s how I think of it.