Simple, uncomplicated sentences are usually unified. They contain just one clear idea.
The old man was dreaming about lions. The old man and the Sea, Ernest Hemingway
The earth seemed unearthly. "Heart of Darkness," Joseph Conrad
Sentences need not be be simple to be unified.Compound sentences can join two clauses effectively. The compound structure says that the ideas contained in the two clauses are related. The conjunction signals the kind of relation that exists between the sentences.
His shoulders were moving, and I wondered if he was crying. "Defender of the Faith," Philip Roth
In complex sentences the subordinate clause expands or explains some part of the main clause.
A blow to my head as I danced about sent my right eye popping like a jack-in-the-box and settled my dilemma. "Battle Royal," Ralph Ellison
Even sentences with very complicated structures can focus on just one idea.
[The whippoorwills] were everywhere now among the dark trees below, constant and inflectional and ceaseless, so that, as the instant for giving over to the day birds drew nearer and nearer, there was no interval at all between them. "Barn Burning," William Faulkner
The complexity of Faulkner's sentence conveys the idea that sounds of the whippoorwill seemed to come from everywhere.