The phrase "a key failing" is correct and usable in written English.
It can be used to highlight a significant shortcoming or flaw in a person, process, or system.
Example: "The project's delay was attributed to a key failing in the planning phase, which led to miscommunication among team members."
Alternatives: "a major flaw" or "a critical error."
Murray couldn't control the ensuing rally, a key failing as the game fell back to deuce.
In the aftermath both Southgate and Dan Ashworth, the FA's director of elite development, pinpointed that lack of big‑game exposure as a key failing.
A key failing of the otherwise world-class Sri Lankan education system is that while many islanders read at least some English, very few Sinhalese can read Tamil, and vice versa.
A key failing, conservationists say, lies in the continent's justice systems, where evidence collection is often botched, prosecutions poorly handled, and judges often don't take wildlife crime seriously, which sends the message that poaching is no big deal.
Not that he approves of the way May has handled the process, identifying as a key failing her insistence on doing things alone rather than relying on her cabinet.
While the new material retains its flexibility in low temperatures, offsetting a key failing of the old O rings, it also melts at relatively low temperatures, raising a new set of fears.
The key failing on this otherwise brilliant record is the lack of tunes.
Another key failing of the articles is that they linked the use of greyhounds in medical research to the cruelty of some practices in greyhound racing.
The site's key failing is the absence in many areas of "alt" tags, text descriptions of maps and images which can be converted into braille.
But the key failing was something different: it was that the credit risks were not distributed.
The randomistas tend to overlook the "key failing" in developing countries, he says: Organizations don't work.
