While reading some manual of PostgreSQL and MySQL (eg. http://dev.mysql.com/doc/refman/5.1/en/upgrading-from-previous-series.html).
I have found that MySQL has stated many incompatibilities and know issues (a long list) in the MySQL version 5.0, 5.1, 5.5, 5.6 and 5.7.
For PostgreSQL, it seems I can't find the list (it just say see the Appendix E / release notes).
I think it is a plus for PostgreSQL if it has few incompatibilities between major versions.
By the way, for in-place major version upgrade (not dumping DB and import again), MySQL is doing a better job in here.
Please share your thought, thanks.