In terms of complaints from the airlines... It has to be delays.
If any delay is caused by the handling agent, they come down on us like a ton of bricks: We're not allowed to make mistakes.
We're in a difficult position. With handling fees being cut down to the lowest possible, we work on a skeleton staff. It usually works fine but given any sort of disruption, we may not have the staff (pax services and loading) to deal with the flights... hence they go out late. The sooner airlines realise that to get a decent service from their handling agent, they need to pay for it sooner or later.
Its a symptom of years of fierce competition between handling agencies who are fighting it out for every contract. In my mind its quite impressive that we've not seen more go bust.
Also if we don't collect enough cash in excess baggage fees we're not flavour of the month either at airline HQ.