Once GPS gets sub-meter accuracy, then we should start worrying.
Sensors designed for computer vision applications are surprisingly light and small! Board-level cameras can fit in the palm of your hand, and reasonable IMUs are not much bigger than a matchbox. The biggest obstacles today are tradeoffs between performance/weight and performance/battery life. The platform needs algorithms with acceptable results, requiring almost no supporting hardware, and that are robust to the thousands of things that go wrong when trying to sense (noise, errors, resolution, occlusions, etc). Algorithms that fit all of these requirements are very uncommon. In fact, I would not be surprised if there was an Iron Triangle in there somewhere.
[0] Inertial Measurement Units: Devices that estimate acceleration (or some other measure of movement).
[1] Simultaneous Localization and Mapping. Using stereo vision, lidar, radio ranging, etc. to estimate your position, plus how the environment looks.
You can get about 1cm of ranging error (median), and say 10cm of positioning error.
Navigational issues resolved, I have a hard time seeing these things having trouble with a modest wind.
A constant wind I agree looks like it would be within their capabilities, though their route planning software would need to take account of it and approach the target from upwind by default - not impossible but another thing to account for.
The real issue for me is gusting. They are definitely small and light enough that power-to-weight issues, particularly when carrying a load with a higher surface area to the wind than the craft itself, means that it will get knocked off course by gusts. That obviously needs allowing for when navigating to and from the construction site, but particularly when dropping off a load - if the release comes at the same point as the gust, the part simply won't end up where they wanted.
Do I think all of these problems are unsolvable? No, but it makes a useful simplifying assumption for their version 1 that they don't have to deal with any of them because they're flying indoors.
Or once the kinect shrinks to size of a short stack of quarters.