Final answer:
SERVICE keyword in SPARQL queries may lead to hard-to-read results due to aggregation of non-uniform data from different external sources and the presence of complex or raw data structures.
Step-by-step explanation:
When referencing the SERVICE keyword, it suggests that you are dealing with a query in a semantically enabled data technology context, such as SPARQL for querying RDF (Resource Description Framework) data. The results might be hard to read because the SERVICE keyword is used to call out to external SPARQL endpoints, and it aggregates results back from different sources which might not have a uniform structure. This can lead to complex, nested data which is hard to interpret especially if the endpoints have differing schemas or the data is returned in a raw format. Moreover, the results might include extra triples or non-human-readable URIs that can make the output difficult to understand without further processing. It is always recommended to use additional filtering, formatting, or post-processing of results to make them more readable.