Installing it before the pressure tank and pressure switch causes rust build up in the water line immediately after the injector and into the switch and its nipple. That in many cases causes problems due to blockage which stops the switch from seeing pressure changes in real time causing a lag in pump operation of both starting and stopping as it should. The cure is to clean the rust out of the line and the switch nipple and the switch which is a real PIA. The frequency depends on the amount of soluble iron in the water.
And since the injector only works based on the pressure switch range, your additional pressure should not be increasing the volume of air as you assumed.