To my knowledge, both Postgres and MySQL has their own strengths and weaknesses. Example: mvcc implementation, data replication, connection pooling and difficulty of upgrades were the major weaknesses of Postgres which are much improved over time. Similarly mysql query optimizer is consider lesser developed than that of Postgres's.

Overall I think Postgres adoption and integrations and thus community is much more wider than MySQL which gives it major advantage over MySQL. Also looking at the number of database-as-a-service companies of Postgres vs those of MySQL we can immediately acknowledges that Postgres is much widely adopted.