It doubles your moving parts
Hear the uncomfortable truth from teams that have actually shipped this. It roughly doubles the moving parts in your system.
You gain an event store, projections, replay tooling, versioning discipline, and lag in your interface, and every new engineer needs weeks to stop thinking in rows. If your domain does not pay for that, the pattern is a tax with no refund, and reversing it later means migrating your source of truth.
Skip it when your data is genuinely current-state shaped. A content system, an internal admin tool, a settings page, an ordinary product with forms.
Nobody will ever ask for the full history of somebody's notification preferences, and a timestamp column plus a simple audit table covers the rare curiosity.
Skip it too when your team is new to it and the project has a deadline. The first event-sourced system anybody builds contains its own set of lessons, and you want to learn those on a low-stakes service.
When it earns its cost
Watch for the specific signals that it earns its cost. Regulators or auditors require provable history, meaning payments and trading and healthcare. Your business asks questions about time: what did this account look like on March 3, and why did the system decide that.
Several teams want the same facts in different shapes, which is the projection fan-out doing real work. Or your domain is natively a ledger, where the events are the honest model and the current state is the derived thing.
Remember the escape hatch from earlier in this course. These are decisions per component, not identities. An event-sourced payments core behind an ordinary product catalogue is a normal healthy system.
The teams that get burned are the ones who made a religion of it and event-sourced their contact form.
Worked example
Two teams at the same insurance company, same year. Team one event-sources the claims engine: regulators demand complete history, adjusters ask what the claim looked like before Tuesday's amendment, and disputes hinge on when facts were recorded. Eighteen months in, replay has saved them twice from projection bugs and an audit passes on the strength of the event log alone. Team two event-sources the internal tool that manages office equipment requests, because the claims architecture looked impressive. They spend three sprints on event versioning for a system whose hardest query is "who has laptop 214," and onboarding a new hire onto it takes a month. In the year-end review, team two's own retrospective recommends rebuilding as CRUD, an estimated two-week job that keeps getting deferred because the event store is now load-bearing.