I have about 200 hrs NVG, so here goes:
All the cockpit lights have to be modified to 'NVG-compatible' which effectively makes all the lights blue/green (with one or two exceptions like the warning lights!). This is because the traditional red/orange instrument lighting appears extremely bright to the NVG. This causes two effects - flaring, which is like a halo around the light source which blocks out chunks of the field of view, and 'backing-off', which happens when the goggles try to compensate for the bright light by 'turning down the sensitivity'(sort of) which makes the rest of the picture go very dim. This is also caused by street lights, security lighting, car headlamps, red obstruction lights, camera flashes, the moon etc etc. The effect is worsened if it is very dark outside. While they are generally invaluable pieces of kit, they are likely to be of little use if operating in urban areas.
NVG cannot be used in complete darkness as they are image intensifiers, not IR scopes. There has to be some ambient light for them to work. Even faint starlight is enough. In an area with no cultural lighting, no moon and thick cloud to block out the starlight, they can be almost unworkable.
To expand the bad weather point, they can actually lead you further into danger than flying without them. They will hide the fact that it is getting misty, or that you are in thin cloud until the point where the picture suddenly goes blank and you are quite definitely IMC, which is not nice if it's winter and you have no icing clearance!
Overall, they should enhance the Police's capability, but I would caution against treating them as the new 'wonder weapon' of night ops. The Police should also brace themselves for a spate of compensation claims for back and neck injuries as a result of wearing them.
(It a lot more complicated than that (lots of boffin techno-wizardry) but I tried to keep it simple)
Hope that helps
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PCD