FHIR should allow components to nest in other components, much like items nest in other items in QuestionnaireResponse. There is no fundamental reason that a component should not have subcomponents.

The workaround available to users in R4 involves using has-member observations, which can be nested to any level. However, one ends up with observations that share all properties (same observer, same method, same effective time, same device, same sample, etc.) that were really part of one observation, represented as separate observations.