Edit:
NgRx effects fire after all reducers for that action have executed. That order is guaranteed. The reduced state is the payload of your effects.
Found the following comment in lifecycle_hooks.d.ts in the NgRx effects-build project:
By default, effects are merged and subscribed to the store. Implement the OnRunEffects interface to control the lifecycle of the resolved effects.
https://github.com/ngrx/effects-builds/blob/master/src/lifecycle_hooks.d.ts
Effects subscribe to the store and they fire when the state store changes. Because we know changes to the state are handled by the reducer, we can reasonably conclude that reducers will always run before the effects.
Also, found an answer from Brandon Roberts, a Google Developer Expert and a member of the NgRx core team, confirming that the order is guaranteed.
https://github.com/ngrx/platform/issues/162