That a spouse will be able to find work once settled. TD dependants may live in the United States and may study, but they are not authorized to work.
The second income that does not exist
This assumption causes more household disruption than any technical failure in a file, because it is discovered after the move rather than before it. A TD spouse may enrol in study and children in TD status may attend school, but neither may accept employment on the strength of the dependant status. A spouse who intends to work needs their own qualifying basis, assessed on their own facts, with its own timeline and its own evidence — and the principal's approval does nothing to advance it.
The question to settle before anything is agreed is what the spouse actually intends to do, and whether the household's budget survives if the answer is that they cannot work for the time being. Remote work for a Canadian employer while living in the United States is not a safe assumption either and should be assessed specifically.