The phrase "a risky assumption" is correct and usable in written English.
It can be used when discussing a belief or idea that carries a significant chance of being wrong or leading to negative consequences.
Example: "Making a risky assumption about the market trends could lead to substantial financial losses for the company."
Alternatives: "a dangerous presumption" or "a hazardous belief".
(That may be a risky assumption on Trump's part).
But this is a risky assumption when you're keeping a car like Jeff's on the road.
She decided to wait until voters started really tuning in -- a risky assumption in itself.
If the rest of Operation Cast Lead goes as well for the Israelis as the bombing campaign – a risky assumption, given the hazards of combat in built-up areas – it is to be hoped that General Ehud Barak is aiming for an early end: some symbolic climax, after which he could declare a victory and pull back.
Mr Clinton also assumes that the European members of the alliance will pay a good whack of the cost; but that may be a risky assumption (France has already said it will not pay anything).Her way of doing things may be especially risky in the Middle East.
But surveys and focus groups assume that people know what is going on inside their own heads - and that is a risky assumption.
But isn't this a rather risky assumption?
A wide-spread concern expressed by the interviewed stakeholders is about the predictability of UNICEF's funding, since the resources that could be mobilized at the local level by regional administrations are believed to be insufficient and, overall, a rather risky assumption.
"It is a careless and risky assumption to make, to presume that U.K. universities would or could continue to benefit disproportionately from E.U. research support" if it leaves the Union, a spokesperson for Universities UK says.
This time, The Sun tabloid has worked a similar vein, though not so stridently, and with the perhaps risky assumption that its readers are familiar with a prize-winning German World War II movie about a U-boat submarine's battle to survive its entrapment by British warships.
Articles often discuss asset accumulation for retirement -- whether historical stock market returns can be expected to continue, for example ("The Risky Assumption in Social Security Change," Economic View, Dec. 26), or whether workers are putting the right amount of money aside ("Before Slicing, Make Sure That the Pie Is Big Enough," Fundamentally, Dec. 26).
