By all means, I am not saying it is fair or right. but as Plazbot says, by the end of your career it all works itself out. Maybe 2 years from now the APP guys are better staffed than the TWR guys, it all goes in circles.
But I will disagree with you in saying that by nature APP is more complex than TWR, and ACC more complex than APP. It all depends on the unit, the airspace, local rules and regulations and staffing. If the TWR is complex, you open ground, delivery, 2nd ground, planner. If APP is complex, you open Departures, Arrivals/director, Coordinator, planner and so on. If ACC is complex you open more sectors, 2 controllers in each sector, planner and so on. At the end of the day, a single controller or sector can only handle so much traffic, and you adjust accordingly.
Having held all three ratings, I would say busy approach procedural is for me the most complicated one, but what does it matter?
Enough drift from the original topic, no it is not fair that some people work more hours/shifts than other people at the same contract.