The phrase "are mostly concerned" is correct and usable in written English.
You can use it when discussing the primary interests or worries of a group or individual regarding a specific topic.
Example: "The researchers are mostly concerned with the long-term effects of climate change on biodiversity."
Alternatives: "are primarily focused" or "are chiefly worried".
In Kenya we are mostly concerned about our own species, the critically endangered black rhino.
As per usual, though, the veteran tinkerers are mostly concerned with matters electronic.
Other people, however, are mostly concerned that their helmet be wired for every electronic accouterment, which is understandable.
Disney is taking a risk by releasing Infinity at a time when parents are mostly concerned with back-to-school shopping.
That is hardline stuff: other immigration hawks are mostly concerned with blocking paths to full citizenship, or ensuring that legalisation goes hand-in-hand with tougher border controls.
Rather, he said, "our members are mostly concerned about the threat of rates going up and some of the most popular tax-extenders going away at the end of the year.
The polls show that Americans are mostly concerned about the economy and that few say their vote will be affected by other issues swirling around the campaigns in recent weeks, including same-sex marriage and the candidates' backgrounds.
A word frequency analysis of the campaign websites for Better Together and Yes Scotland shows they are mostly concerned with the same issues: currency, tax, the EU, and oil.
WHEN people in Washington talk about shrinking the drone program, as President Obama promised to do last week, they are mostly concerned with placating Pakistan, where members of the newly elected government have vowed to end violations of the country's sovereignty.
For one thing, as Matthew Yglesias pointed out, even on the social issues that liberals and libertarians agree upon, liberals often want to use the power of the state to advance their agenda, where libertarians are mostly concerned with preventing the state from discriminating.
In his book about Runyon Heights, "Red Lines, Black Spaces: the Politics of Race and Space in a Black Middle-Class Suburb," Mr. Haynes wrote, "Residents are mostly concerned with maintaining good schools, safe and clean streets and the middle-class suburban lifestyle".
