"Air Duster" sprays - are they safe for hardware?

Yes it is safe to spray compressed air into electronics as long as the electronics are OFF before you spray into them. Be sure to hold the compressed air can vertical while pressing its 'air-release'.

If you hold it at any other angle than perpendicular to the ground, liquid can omit from the can (which can take a few minutes to evaporate)

The can must be held upright during use. Inverting, tilting or even shaking the can during use may result in the unevaporated liquid being forced through the nozzle instead of the gas. The liquid will boil away almost instantly outside the can, producing extreme cold in the process. In liquid form, the contents of the can will act as a solvent, causing unwanted damage to surface coatings or labels, this is generally only a problem with optical lens coatings. Side effects of the intense cold can also cause problems due to localised condensation.

Source

A bigger problem could be water. Depending on the humidity of your environment, you could also have some condensation forming. Sudden decompression drastically lowers the temperature of the canned air , which could cool the components and allow some of the moisture in the air in the room to condense on them.

You'll always notice a drop in temperature, but condensation will form only if it's quite humid. If you believe this could be a problem, test it on some non-critical surface and then touch it with your finger or a paper napkin to see if any condensation has formed.

When I first used a can of Dust Off in a humid outdoor environment (on the interior of a mechanical film camera, luckily), I was quite taken aback by the size and number of water droplets that formed.