I'm going to clarify a couple of points here in bullet form:
1. Engagement Score takes 72 hours to calculate, whether it's a program or an email in the stream
2. We cannot calculate the unsubscribe rate for an email inside of a program at this time. This is an enhancement. So it is not in error that you see 0%
3. Programs give you more control over when to send a piece of content again or not - if you add a piece of content to a stream, and it already has members, then if the lead is a member already, it will skip it. If there are no members, then it looks at whether or not the lead ran through it. If you use the same program across multiple streams, it will not send the content more than once.
Hope this helps clarify!