* Cost

  Running costs: Plain EC2 DB is cheaper than RDS. RDS is instance costs plus RDS tax. 

  Configuration and maintenance cost: RDS obviously cheaper

* Performance

  As other replies have detailed plain EC2 with NVMe etc could be significantly faster.

* Convenience

  RDS wins most of the time.

* Risk

  RDS wins unless heavy investment

So it depends on your requirement. If high performance with massive DBs is important, then the costs of managing your own DBs may make sense.

If a low throughput, less risky DB then your own DB may make sense.

If a normal business use DB, outsourcing the maintenance and risk to RDS may make sense.