1. We (an NFL team) had an Oracle 9i database (which Oracle pulled support for in 2010) storing NFL player's Scouting information for > 20 years.

2. We needed to make the switch to a modern solution because as of June 2016, we had been running w/o support for 6 years (I got here in 2016), and the RDBMS (9i) was quite slow and not compatible with modern drivers and applications.

3. Our biggest technical hurdles were unique to the tool that we used for migration (etlalchemy, described in (4) below).
i.) Finding the correct cx_Oracle (Python) driver to communicate with the correct Oracle Instant Client version, and then finding the exact version of Python libraries which supported that driver (b/c Oracle pulled support for 9i so long ago, finding the old instantclient binaries was nasty work).
ii.) Handling FK constraint violations when importing data into PostgreSQL. Because you can't turn constraint checks like you can on MySQL (SET FOREIGN_KEY_CHECKS=0;), data that violates constraints must be resolved before the migration. As a solution, I added support to etlalchemy (the next bullet below) to "dump bad rows" to STDOUT, and skip the foreign key that is violated during the "constraint migration" step.

4. Regarding tools, we used https://github.com/seanharr11/etlalchemy to facilitate the migration. I created this open source tool 2 years ago, published it to github, and used it to carry out 99% of this project. The only thing that had to be migrated manually were the Oracle functions (most of which we tossed out as they were used for ColdFusion/Report Generation). The most powerful feature of this tool is its ability to migrate schema (including column types) between 2 different RDBMS's. It also loads data quite fast leveraging PostgreSQL's 'COPY FROM' bulk import, and MySQL's 'LOAD DATA INFILE'. (Constraints and Indexes are also migrated with etlalchemy out-of-the-box)

5. From a business standpoint, we are just completing the project now and are likely going to choose MySQL as our final destination. This is mainly because our database is very OLTP-oriented. Our applications perform very small SELECTS, INSERTS and UPDATES to our database at high frequency (think football scouts updating player statuses, live stats feeds from the NFL, etc...), and InnoDB (MySQL storage engine) happens to do a very good job of facilitating this type of database (high-frequency simple updates). PostgreSQL would have been a better choice if we were to run less frequent queries involving more computation (OLAP-oriented), but we don't (yet) have a huge Machine Learning/Data Mining requirement here. From experience, PostgreSQL has a huge toolkit of functions, very cool indexes, and other nice-to-haves, we just don't need them for our business case. Both choices have a much stronger community, and answers are found via StackOverflow/Google rather then reading the Oracle manual until your eyes bleed.

6. If you want to read about my experience, I have posted a quick write-up here, including some more background info on etlalchemy: thelaziestprogrammer.com/sharrington/databases/migrating-between-databases-with-etlalchemy

If you decide to give the tool a try (it is battle-tested), please let me know how it works for you. And please, if you have any questions/feature-requests/issues, reach out to me. I am trying to get some momentum going for this project, as it would solve lots of people's database migrations problems as it solved mine.