"formidable effects" is correct and usable in written English.
You can use this phrase to describe the visible and often powerful effects of a particular event or action. For example, "The formidable effects of the storm left the town in ruins."
But her vivid sartorial choices are far from incidental to the formidable effect of her playing.
Familie Flöz is a Berlin-based company that uses masks to formidable effect, and its Teatro Delusio (Pleasance) attracts a deservedly packed house.
Walter pulled the strings to formidable effect; no fewer than four of the Germans' half dozen goals came from set pieces, and he coolly scored twice from the penalty spot.
The election year brought portraits of the two presidents who might legitimately be called America's greatest: Franklin D Roosevelt was impersonated by Bill Murray in the light-hearted Hyde Park on Hudson and Daniel Day-Lewis portrayed Abraham Lincoln to more formidable effect in Steven Spielberg's film, about to open here.
Prices are down 38% from last year, based on Trulia.com data from California's multiple listing services, and the state of the housing market has had a formidable effect on homeowners' net worths: with home equity at just 19% of home value, it's the most depressed housing market in the country.
A mere letter supporting or opposing a regulatory policy can have a formidable effect if the right names are on it.
The formidable side effects of individual or combined anticancer agents as well as the treatment history and clinical status of the patient must be considered to circumvent the tumor resistance to chemotherapy in clinic.
These byte-sized actions reverberate throughout the social graph, resulting in a formidable network effect of measurable movement and activity.
The Statusphere is the new ecosystem for sharing, discovering, and publishing updates and micro-sized content that reverberates throughout social networks and syndicated profiles, resulting in a formidable network effect of activity.
