If my GPS experiences are anything to go by, big stuff (buildings and mountains) and cold (possibly heat too?) make GPS inaccurate enough that it would be pretty much useless. Up high in subzero temperatures on a mountain we would get strange coordinates back due to (I assume) the mountain blocking or reflecting signal somehow. And that was when it worked - sub zero with a big wind chill and the batteries would last about 10 seconds, then back under the jacket it had to go to warm up. This has been on multiple brands of hardware. Things may have improved since this experience as it was 5 years ago.

I doubt GPS could be trusted for autopilot, but if the accuracy is reliable for a ~30 foot sphere it could be used as a proximity alarm. ("Cool your jets human, there's another drone nearby")

The GPS on my iPhone is good enough to pinpoint my 2D position to within ~10 feet, so I don't see why not.

GPS is good enough for autopilot; I've flown with them just fine. It's also common to do GPS approaches in inclement weather, and when you break out of the clouds, the runway is right where it should be!

1. It's Australia. It doesn't snow here. 2. Almost all apartments have balconies. Good luck landing on the 10th floor during storm season though.