"unprecedented setback" is correct and usable in written English.
It is used to refer to something that is unexpected and serious, something that has caused harm and is difficult to overcome. Example: The company faced an unprecedented setback when its technology was hacked, resulting in the loss of proprietary data.
Yet while the legalization movement is currently enjoying unprecedented momentum, the setbacks and backlash that it and other social movements have faced since the 1970s suggest that continued victories are far from guaranteed.
But Nazi ideas of Aryan supremacy were dealt a severe setback by the unprecedented four gold medals won by African American runner Jesse Owens at the 1936 Olympics in Berlin.
The process involves a wide-ranging review of progress made for children since 1990 - and there the leaders will find a picture that is decidedly mixed: unprecedented success mingled with dispiriting setbacks.
With his live-streamed, interactive Intel powered "Electrified" stunt kicking off in New York, endurance artist David Blaine joined What's Trending for an exclusive Google Plus live hangout to reveal his preparation, setbacks and expectations for the unprecedented event.
The decisive support for Juncker after an unprecedented vote among EU leaders for a commission president marks a major setback for Cameron, who had thought key EU allies would oppose his nomination.
Also in Colorado, the petition-gathering effort for the Mandatory Setback from Oil and Gas Development Amendment (Amendment 78) was a target of an unprecedented "decline to sign" campaign, which was funded by the amendment's fossil fuel company-backed opposition.
But this year there have been setbacks which have threatened to put the brakes under his unprecedented era of dominance.
The hacked emails of campaign chairman John Podesta, being released daily by WikiLeaks, have offered unprecedented insight into the way the Democratic nominee and her team grappled with unexpected developments and self-inflicted setbacks.
