"knotty tasks" is a correct and usable phrase in written English.
It can be used to describe difficult or complicated tasks, especially ones that involve a lot of problem-solving or untangling of complex issues. Example: "As a team, we tackled the knotty tasks of implementing a new software system and restructuring our workflow. It was a challenging process, but we were able to successfully navigate through it together."
The knotty task of integrating two huge research teams can distract both managers and scientists from the all-important quest for new drugs—a problem that will not show up in the balance sheet for several years.
In tray: Tough, feisty and straight-talking, her job will be the knotty task of cutting regulation to promote small business growth – without falling foul of EU employment laws.
Iraq handed over to the United Nations office here today a list of more than 500 experts involved in the development of ballistic missiles and nuclear, chemical or biological weapons, setting the stage for one of the knottiest tasks facing the renewed inspections.
If you are a regular user of the Internet today, you take all these knotty engineering tasks for granted.
Instead of reciting the well-known litany of human rights abuses that defines contemporary refugee policy, the panellists on offer were tasked with presenting potential solutions to this notoriously knotty corner of Australian politics.
At work I manage to concentrate on other wordsmith tasks related to my editing job, but sometimes a particularly knotty clue will follow me around all day like the Cheshire cat, gleefully elusive.
Together they explore the strange sensations of rain and the knotty furrows of trees, overcome almost insurmountable hurdles to perform "simple" domestic tasks (watching them change a lightbulb is more dramatic than you can possibly imagine), and write plays, prose and poetry that offer others an insight into their seemingly limitless world.
Meanwhile, the mundane and frustrating task of dissuading or eliminating Islamic terrorists still has problems knotty enough to make chumps of budget and technology.
Other knotty issues abounded.
