Use a Cases with Milestones report filtered on Violation equals True when you need to know which SLA step was missed and by whom. Choose a plain Cases report filtered on Milestone Status the moment a manager wants one row per late case, and fall back to case age only until entitlements are live.
I built all of this in a Developer Edition org for two customer accounts, Ridgeline Supply and Lakeshore Health. Five of eight cases missed their first response target, and both milestone-based reports found exactly those five. By the end you will know which SLA breach report in Salesforce fits your team, how to build it, and the four settings that quietly hide breaches.