Virgin,
NVGs are a tool like any other and have their limitations. Like most people I wore them for the first time and thought - WOW look at all the things I can see. After you've used them for a while, and if you have good instruction, you begin to think - oh dear, look at all the things I can't see.
Peripheral vision is non-existent and it takes conscious effort to move your head around and not fixate on the view in front. Depth perception is poor and you tend to find yourself underestimating the distance to obstructions. Apart from on the brightest nights you can't see wires, cables and whip ariels, the biggest helicopter killers, so good map reading is essential. You can see through most types of precipitation fairly well, so it's not unusual to look under the goggles on what you think is a nice night to see the windows covered in rain or snow. If you have a goggle failure or approach a well lit area where the goggles won't work you can find youself suddenly trying to fly visually, at night, in 500m vis in mist.
Having said all that, NVGs are still a fantastic tool for helping you fly at night and I wouldn't be without them if I could help it.