The phrase "are completely capable of" is correct and usable in written English.
You can use it to express someone's or something's full ability to perform a task or function.
Example: "The team members are completely capable of handling the project on their own without any external assistance."
Alternatives: "are fully capable of" or "have the ability to".
Believe it or not, young ladies of 11 are completely capable of behaving in a catty manner without an extra push from you.
Sure, I've taught my kids about gun safety, but their brains aren't fully developed to the point where they are completely capable of formulating a rational plan for dealing with a dangerous weapon they might accidentally encounter.
"They are completely capable of going back to work.
The walls are completely capable of holding themselves up and do not need your assistance so step away from them.
For Holder and the Justice Department, the Khattala case offers another chance to prove that federal courts are completely capable of handling complex terrorism cases.
Although you may not feel it now, you are completely capable of retraining your thought patterns to support a happier, more energetic, and more successful life, and it's actually much easier than you might think!
The Vamp, who'd detested her stepson, was completely capable of doing him grievous bodily harm, as the chief would say, in his detective-movie way, and then making her daughter feel guilty for it.
And not having a bunch of grown-ups doing stuff for you when you're completely capable of doing it yourself, and being able to take care of your own business.
Sure I'm completely capable of brushing my teeth manually, but why?
Already, the team has developed a simple test that seems to be completely capable of weeding out the bad clones very early, perhaps even at the tissue culture stage.
Sometimes it makes other burrows from other bigger animals, even though it is completely capable of digging one on its own.
