"Nihongo wa hanashi nikui" is incorrect.

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Have you ever said, “Nihongo wa hanashi nikui” or “Nihongo wa yomi nikui”? Every time I hear those, they feel odd to me. I often come across incorrect usages of “nikui” in other situations too. The reason is simple: you just understand “nikui” as being the same as “difficult to do”.
But that alone isn’t enough. There is one more condition necessary for correct usage. Any ideas what that is?

For example, what is it like eating crab? Sure, it tastes good, but you have to break or remove their shells, so it’s an annoying food.

A: kani wa oishii kedo, tabe nikui.

Consider this next example: You can’t eat any more because you’re full but, despite this, food is still being served.

B: onaka ga ippai dakara, zenbu taberu no wa muzukashii.

Example A refers to crab being difficult to eat because of the shape and shell. It’s that feature of crab itself that causes problems.

For example B, the food itself doesn’t have any problem. You are simply full and can’t eat it. The only issue is your own ability or, in other words, capacity.

Here is another pair of examples.
Japanese people often say something like this after the first sip of wine or sake.

A. kono wine wa nomi yasui ne.

This means that you can drink the wine because it tastes good.

On the other hand, when you need two people to drink a bottle of wine because you can’t finish it off on your own, you might say,

B. futari nara wine o ippon nomu no wa kantan des.

It’s not the quality of the wine itself that matters here. It’s your own ability or capacity to drink it that does.

Therefore, ”… nikui” (A) is used when a characteristic of something makes your own actions difficult and “… yasui” (A) is used when it’s easy for you to do something with the item because of its inherent characteristics.
Meanwhile, “… no wa muzukashii/ kantan” indicates that you yourself have or don’t have the ability or skill to do something. Also, external factors can affect things sometimes too.

Let’s think about this from a different standpoint.

While you are drinking wine, let’s say you spill some on a table cloth.

A: Wine no shimi wa ochi nikui.

B: Wine no shimi o otosu no wa muzukashii. 

Example B means that the stain isn’t likely to come out because you don’t have the knowledge or know the technique to wash it away. “Otosu” (wash away) is your own action, so we can see that any verb used here must be a transitive verb (a tadoushi) or else an intransitive verb (a “jidoushi”) describing someone’s action, such as “iku”, “hashiru”, “hairu” and so on.

Lastly, I will write about “nihongo wa hanashi nikui” and “nihongo wa yomi nikui”. I wasn’t able to give a good explanation about why they are wrong, but now I think I’ve come to a conclusion. Here is my interpretation.
The Japanese language is very different from Western languages, and hiragana, katakana and kanji make it complicated. It’s true that non-Japanese people have difficulty speaking and reading Japanese and it seems to them that pattern A should be possible. However, try thinking about it this way:
For Japanese people, the nature and characteristics of Japanese are normal and not difficult. Also, they would never say “nihongo wa hanashi nikui”. Considering this, it seems Japanese people have a tendency to think that the difficulty of a language is more related to individual skill or ability than the characteristics of the language itself. It's a subtle but important difference, I think.

However, “hanashi nikui” could be used in the following situation:

“ano hito wa itsumo okotteite, kowasou dakara, hanashi nikui na”.

It’s hard to talk to that person because he is always angry and looks scary.

In my opinion, “hanashi nikui” is used like this and is not used to refer to the difficulty of a language.

What do you think? Are you convinced?