The phrase "a key difficulty is that" is correct and usable in written English.
You can use it when introducing a significant challenge or obstacle in a discussion or analysis.
Example: "A key difficulty is that many students struggle to balance their academic responsibilities with their personal lives."
Alternatives: "a major challenge is that" or "a significant issue is that".
A key difficulty is that stakeholders typically convey their goals indirectly through intuitive narrative-style scenarios of desirable and undesirable system behaviour, whereas goal refinement methods usually require goals to be expressed declaratively using, for instance, a temporal logic.
A key difficulty is that Mei Xiang places herself in what he called "pancake position" — flat on her stomach, legs outstretched — and Tian Tian isn't assertive enough to lift her off the ground.
A key difficulty is that this plot is so predetermined there is little point in seeing it through to the end.
However, a key difficulty is that we can only observe three types of reference components due to the limitation of staining and imaging techniques.
Dyke said a key difficulty was that most observers believed that the BBC Trust would cease to exist, or at least be radically overhauled, as part of charter renewal in 2016.
The key difficulty was that validation was only possible in a relatively small subset of the population who actually reported an event.
The key difficulty is that cancer is a complex and heterogeneous disease: many genes are amplified, deleted, mutated, and up- or down-regulated.
The key difficulty is that operating in everyday listening conditions entails processing a speech signal that is reverberantly mixed into a noise background composed of multiple competing sound sources.
A key source of the difficulty is that most unobserved factors that could affect callback rates should theoretically affect offer rates conditional on callbacks in the same direction.
"A key thing is that presentation.
A key difficulty in addressing this problem is that expansion of biomedical information is asymmetric – different areas of knowledge grow at different rates which in turn vary (and are ascribed differing priorities) during different periods (Fig. 1B).
