What happened to the Unique Events metric?
The Unique events metric featured in Universal analytics is based on a session model, e.g. how many times an event fired per session.
The App+Web measurement moves from a session-based to user-based model. Therefore, this metric is not available in the reporting interface.
App+Web data is streamed into BigQuery so you could calculate it based around the session_start event if you wanted to see this metric ongoing.