The phrase "difficult to illustrate" is correct and usable in written English.
You can use it when referring to something that is not easily depicted with words or images. For example, "The complexity of the human mind is difficult to illustrate."
It would be too difficult to illustrate what's real and what's not real.
Programming advances also get much shorter shrift than breakthroughs in hardware, perhaps because they are more difficult to illustrate.
The danger of dioxins in our environment, our food chain and our bodies is difficult to illustrate, since they are not visible to the naked eye.
The third important structural model, the random close-packing model for metallic glasses, is difficult to illustrate with a simple diagram.
It proved no less difficult to illustrate the more scandalous moments in Nijinsky's life, not least the near-riot that erupted at the premiere of Nijinsky's monumental "Sacre du Printemps," set to music by Stravinsky, at the Theatre des Champs-Elysees on May 29 , 1913
Since then the reputation of much of the architecture of the 1950s has been rehabilitated, but his clamshell seems doomed to be an architectural untouchable — it is difficult to illustrate the incremental insults to Penn Station, the ticky-tacky additions, the grimy travertine, but a photograph of Tichy's structure is always ready to stand in.
It would be a difficult shot to illustrate and animate believably from the imagination alone, so lilfuchs resorted to extreme measures, using his family as models.
Along with his rival, Iori, Kyo has been noted by several of the series' designers as being one of the most difficult characters to illustrate because of his popularity amongst fans.
Even difficult-to-illustrate situations can thus be reproduced with the help of videos.
Intuitively, almost half of the words in the vocabulary seemed to be difficult to be illustrated by sound.
The latter is quite difficult to be illustrated graphically within a total legend for the electron density, but it follows directly from the data on change in interatomic distances in the carbyne chain given in Figs. 4 and 5. Thus, the contact bond instability due to thermal fluctuations induces the release of elastic energy accumulated in the loaded crystal, which facilitates the chain break.
