stats and eval are recommended over subsearches because they are more efficient and scalable. Subsearches can be slow and resource-intensive, whereas stats aggregates data, and eval performs calculations within the search.
The stats and eval commands should be used instead of subsearches whenever possible because subsearches have performance limitations. They return only a maximum of 10,000 results or execute within 60 seconds by default, which may cause incomplete results. Using stats allows aggregation of large datasets efficiently, while eval can manipulate field values within a search rather than relying on subsearches.
Reference:
Splunk Documentation - Stats Command
Splunk Documentation - Eval Command