Fully managed services reduce your operational overhead as AWS manage not just the infrastructure
layer but the service layers above it. Examples are Amazon Aurora and Amazon ElastiCache where
the database is managed for you.
You do not have greater control and flexibility with fully managed services. AWS take more
responsibility for providing the service and you therefore have fewer options. For example you
may not be able to configure the performance parameters of a database as you’d like to or use
your own backup or operational software.
Security is not necessarily improved by managing your own software stack. AWS are extremely good
at securing their services and there is arguably less chance that they will expose
vulnerabilities than a customer who deploys their own applications.
You do still need to backup your data. For instance, with Amazon ElastiCache it’s up to you to
configure backups to S3.