The phrase "a key difficulty" is correct and usable in written English.
You can use it to highlight an important challenge or obstacle in a discussion or analysis.
Example: "One key difficulty in implementing the new policy is the lack of resources available to support it."
Alternatives: "a major challenge" or "a significant obstacle."
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.
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 for home buyers in New Jersey," she said, "has been that a lot of the service people they call don't show up -- or the attitude is 'I'll be out when I can.' "We continually check to make sure the people on our list are reliable and responsive," she added.
The highly dynamic and stochastic nature of ATM traffic poses a key difficulty for this tradition.
Characterizing the set of such equilibria is a key difficulty in studying this problem.
For the latter, lack of relative motion is identified as a key difficulty in the association process.
Because cloud computing addresses a key difficulties surrounding large scale data processing, and because there are many difficulties that current technology is facing with there is a need to find ways improving the services to the client by introducing new techniques for upgrading the service.
What was going on behind the scenes in Copenhagen is a reflection of the key difficulty facing those who want to secure a new global climate treaty - what is the ultimate priority for nations?, cutting emissions of carbon or lifting people out of poverty by whatever means necessary?
The key difficulty in conducting an IV analysis is finding and justifying a strong instrument, especially if the research questions revolve around multiple instruments and predictors.
The key difficulty in designing a program of this type is to preserve its simplicity.
While a number of organizational and cultural issues are involved, a key technical difficulty lies in linking together the different time and length scales at play into a single representative format.
