It is true that learning to read the kanji is harder than learning to read, e.g., Spanish. But I understood your original statement to not be about ease of learning but ease of reading (once learned). It is not obvious to me that Japanese is at a disadvantage here when we compare kanji vs. romaji within the same subject.
By the way, you might know this, but if a Japanese speaker doesn’t know a kanji, they are not at a complete loss. Words often consist of multiple kanji so they can infer from the meaning and possible pronunciations of the other kanji in the word. The context in which the unknown kanji appears helps too. And in literature aimed at young readers, they often also show the pronunciation written next to more unusual kanji (furigana). All of this is less relevant to APL though.
Usually hard to learn and hard to use are very correlated concepts. If something is hard to learn, it usually requires more mental energy to read. For an example closer to APL, mathematical notation is harder to read than the verbose equivalents. You not only need to keep the concepts in mind but also how the symbols are translated to the concepts. Maybe when you’re very familiarized with them they’re as easy and a little bit more terse, but it’s easy to slip back if you’re tired or just woke up less bright that day.
And that’s just in math, where there isn’t as much symbol density as in APL and where symbols don’t depend on context too much (it’s usually not a good practice and avoided if possible to have the same symbol having very different meanings depending on the context).
By the way, you might know this, but if a Japanese speaker doesn’t know a kanji, they are not at a complete loss. Words often consist of multiple kanji so they can infer from the meaning and possible pronunciations of the other kanji in the word. The context in which the unknown kanji appears helps too. And in literature aimed at young readers, they often also show the pronunciation written next to more unusual kanji (furigana). All of this is less relevant to APL though.