[knute]

There are a limited number of spy satellites, and their orbits are fairly fixed. If you need to look at a site you have to wait until a satellite passes it (and the people at the site will be able to know quite far in advance when a satellite is coming). And there's no way to delay the pass until the cloud cover clears.

[sterlind]

You could potentially hide a satellite from radar observation the same way you can for planes. That's apparently what the Misty program [1] did. Synthetic aperture radar could take care of clouds, though you lose color. Agreed about maneuverability I think.

1. https://en.wikipedia.org/wiki/Misty_(satellite)