I think you have to give some examples.

It's been a while, but the one I remember with the most horror was dealing with Enums, and Enum payloads. Since Swift is statically-typed, many 'type union' situations end up becoming difficult problems. The question usually boils down to thinking about choosing one of (a) Enum with Payload (b) Generic (c) Type erasure. Nearly anything technically is do-able, but I wasted many hours (ie: repeatedly, not until I figured out the 'right way' for every situation) deliberating over how!