Pitchlink,
The general rule is that the checkflight determines the airworthiness of the aircraft for normal service, and that some small but measurable risk is incurred. I wrote that procedure, mostly, and didn't want to tie anyone's hands, but on the other hand...........
If you are at night or IMC, there is less chance for a successful precautionary landing.
I know of one fatal where a night maintenance test flight ended in catastrophe, perhaps due to crew distraction and weather.
OTOH, I must have flown off 20 engine changes in Vietnam after midnight to prep for the next morning's missions.
The flight check manual is not law, it is the manufacturer's recommendation.