When you are doing database migration, what is important is which database you want to migrate to and which database you are migrating from. If, say you want to transfer a bulk of data efficiently, it is not recommended to go for Nifi, but rather the ones like Sqoop.
Apache NiFi is a tool to build a dataflow pipeline, it is just the right tool for Internet of Things (iOT), Internet of Everything (IoE) or any data in motion using inbuilt connectors (also known as processors in NiFi world) so it can Get/Put data from/to HDFS, Hive, RDBMS, Kafka and many others. It also has a really cool and user friendly interface which can be used to build the dataflow in minutes by dragging and dropping the processors. Nifi is great for just about any simple event processing use case, but for a large scale RDBMS ingest it is not so useful.
Learn more about NiFi dataflow automation and streaming analytics by joining Apache NiFi Training and tutorial.