The correct service for uncovering sentiments, entities, and key phrases in unstructured text is Amazon Comprehend. It is designed to analyze text and provide valuable insights without the need for deep expertise in machine learning. Amazon Polly is incorrect because this service converts text to speech and does not analyze text content. Amazon Translate also does not fit as it focuses on converting text from one language to another and does not perform in-depth text analysis. Lastly, AWS Lambda is a compute service for running code in response to events and does not provide text analysis capabilities.