The DDs have a bad rep from their unreliability a year or two back, alongside the preference toward using ICRs for late autumn/early winter. Part of it is just the fact of life of loco hauled v railcars. A railcar with an engine (or two) down can limp home, a loco down and the train needs to be rescued and a loco found at short notice to do so. This was less of an issue when there was locos all around the system, that is not true today. Many a time a 29 is used to haul a Dart up to Fairview!
In terms of comfort, DDs are miles ahead. If the ICRs had seating as comfortable it'd be the cherry on top of what is otherwise IMO an excellent, reliable class. It's agony trying to get comfortable in them!