Halliday & Matthiessen (2014: 715):
… the metaphorical mode thus makes available a great deal of further ideational potential that is not accessible in the congruent mode. At the same time, the metaphorical mode also denies access to significant aspects of the potential that is associated with the congruent mode: there is a loss of ideational meaning. For example, the tactic patterns of clause complexing (with the distinction between paratactic interdependency and hypotactic dependency) are not available to sequences that are realised metaphorically as clauses, and the configurational patterns of participant roles are lost or obscured when figures are realised as groups or phrases. Thus in the nominal group the perception of an inadequate retirement program, it is likely that an inadequate retirement program is what is perceived, but this has to be inferred (contrast the perception of concerned citizens, where concerned citizens is likely to be the perceiver, not what is perceived); and the perceiver is left implicit. In more congruent versions, such underspecification does not occur: people see the retirement program as inadequate, people think that the retirement program is inadequate, and so on. This is because the grammar of the clause construes participants as inherent in the process, and only allows them to be absent through ellipsis if they are recoverable.