The phrase 'is only modestly' is grammatically correct and can be used in written English.
It usually means that something is only slightly or moderately something, and can be used to express a lack of emphasis or a more reserved opinion. Example: "The performance of the team was only modestly successful, as they were not able to secure a win against their toughest opponent."
The F.D.A. decision was not totally unexpected since Contrave is only modestly effective in helping people lose weight.
Only one drug has been approved to treat it, Rilutek, made by Sanofi, which doctors say is only modestly effective.
Growth is only modestly below the trend of the past few years, although the danger of inflation remains, it noted.
Yet his chief campaign issue is only modestly liberal: preserving the government's system of subsidised student aid for college.
S9 BASEBALL -- Phoenix, where the World Series begins on Saturday, is only modestly anxious about the possibility of a terrorist attack.
The situation is only modestly better in Sydney, Australia, where the government spends $46 million a year maintaining the sites of the 2000 Olympic Games.
The first Aids vaccine to show an encouraging result in clinical trials is only modestly effective and did not protect those at highest risk of HIV, it emerged today, as US and Thai researchers revealed their full results in Paris.
Some 42% believe that such a top wage is either acceptable (36%) or too low (6%), a large minority that is only modestly outnumbered by the 47% who believe that this sort of pay gap is excessive.
The evidence that Mr. Romney's bounce is receding some is only modestly strong — as opposed to the evidence that he got a significant bounce in the first place, which is very strong.
Yet the Mayo Clinic overall is only modestly profitable, according to its financial statements.
Furthermore, a large portion of patients are still treated with conventional chemotherapy that is only modestly effective.
