Any thermal device, be a scope or a handheld or vehicle mounted device, will work without hesitation during any lighting conditions. The only difference is the daylight can make things hotter than they would be at night, things such as rocks, concrete, trees, ground, etc can all gather thermal energy during daylight which will give a slightly skewed view compared to the dead of night. However, I have never missed game sized animals due to these types of anomalies, because animals (warm blooded of course) always have more thermal heat than their surroundings.
Night vision is a totally different beast. True/conventional NV devices will get ruined if exposed to too much daylight as the light will burn out the NV elements. With the advent of digital NV devices, you don't have to worry about exposure to light and can use them during the daytime, but their use is limited big time as everything seems bleached out due to too much light. With proper atmospheric
Conditions (overcast/dawn/dusk) you can use digital NV quite well with proper settings adjusted. But in bright clear skies, you would be better off shooting nearly blind.
The X-sight linked above is advertised to have a digital daytime display with full color and a digital NV display with the flip of a setting. Could be a great little gizmo, problem is we are still waiting for them to be released.