NiFi looks very promising for numerous chunks of the ETL capability space, but I'm not sure it can stand alone on the transformation piece. I've been looking into coupling it with python for a full ETL stack.
JPKab - As you explore NiFi more and pair it with your own scripts I'd be curious to hear if you think there are things we can and should do better to be more complete. Let us know at dev@nifi.apache.org Good luck!
Alternatively, you might be interested in Apache Nifi for ETLs (apparently used by the NSA for big data stuff...) then combine with Metabase.