The phrase "unjustified assumptions" refers to ideas or beliefs that we accept as true without any proper evidence or reasoning to support them. It means making assumptions or conclusions without a valid and logical basis. Full definition
You make unjustified assumptions and derive spurious results from them. (judithcurry.com)
Thus, S&W make a number of unjustified assumptions and sweeping statements which turns it into a mere speculation. (realclimate.org)
You must present full equations without unjustified assumptions that any particular variable does not depend on x to get correct equations. (judithcurry.com)