Review estimates compared to actual delivery
Using Atlassian JIRA - Create a report showing each assignees JEI (JIRA Estimate Index).
The JEI is calculated using the original time estimate divided by the logged hours on that issue.
A number < 1 = More time spent than estimated.
A number > 1 = Less time needed than estimated.
You can dynamically add/remove epics in the report to see how that affects the JEI of each assignee.
An epic with a uniform low JEI value can be an indication of a overly complex issues or poor acceptance criteria.