Step 1: The femoral neck sits inside the hip joint capsule, and its blood supply through the retinacular vessels is easily torn at the moment of injury. This anatomy explains why its complications are unusual.
Step 2: Because the dead-end blood supply is so fragile, the head frequently loses its circulation, giving avascular necrosis - the single most frequent problem. When the fragments fail to knit, non-union results, which is the next most common issue.
Step 3: Over the following years, a head that has gone avascular or collapsed wears out the joint surface and produces secondary osteoarthritis of the hip.
Step 4: A united but deformed position, that is malunion, requires healing within surrounding soft tissue and periosteum. An intracapsular fracture lacks this, so malunion essentially never happens here, making it the exception.
\[\boxed{\text{Malunion is not a complication of fracture neck of femur}}\]