Is it advisable to mount an outdoor air-conditioner on a wall with brackets?
An air conditioner can be installed on pretty much any brick, concrete or block wall. As long as it is installed properly, there is no issues to it falling or structural integrity of the building. Air conditioners are mounted on brackets all over the world, especially where space is a premium. You can always place rubber pads between the brackets and the unit to help dampen any vibration. I suggest gently having the technician place 1 screw on either side of the brackets and condenser to keep it from being knocked off easily. 

A properly leveled and mounted condenser will transfer the same amount of vibration into the building as one on a patio stone, as long as there are no issues with the unit. I find Majority of condenser/compressor vibration and noise is transferred into the home from the line set and not the brackets.