Goggles do need some amount of ambient light to work...and they current models are limited to a 40 degree field of view (though research is being done with 110 degree panoramic NVGs). However, current models are capable of 20/25 visual acuity as opposed to best case 20/200 with unaided night vision.
Cockpit light compatibility is another issue...non-NVG compatible lighting inside the cockpit will gain down goggles so much as to make them unusable. The most predominant compatible lighting is blue/green however I have recently heard of white NVG compatible lighting -- an EMS operation in Asheville, NC recently had their bird outfitted for about $12,000 USD.
And as for weather, NVGs can definitely get you into bad situations -- especially in a "pervasive" precip environment like drizzle, rain, & snow. However, in other areas like mountains, desert, etc. you can see low lying fog or rain shafts and maneuver around them. I've used NVGs to direct a USMC KC-130 aerial refueling tanker that was unaided around a rain shaft to avoid us all going into the clouds. Just my $0.02!