I realized that it's not that accessors and mutators are problems in themselves, but ones that return primitives (strings, integers, etc.) are. Returning complex objects is something you do all the time, and it doesn't matter if the method happens to have the word 'get' or 'set' in it

Just a point of clarification: Get/Set methods are a short way of labelling methods that access an objects attributes, that is, methods that are only concerned with reading or writing object state directly. It has nothing to do with whether the value of that slot is a primitive or an object, but whether it represents object-state. Objects should rarely provide methods to the outside world that are only concerened with accessing state.