Surprised by that. I've used AWS DMS quite a lot to do both on-prem to AWS and AWS (MySQL) to AWS Postgres migrations and long term ongoing replication. Whilst there is some complexity/gotchas there it's always been more than up to the task. Takes a little bit of validation/testing to understand but it's very well documented too.
What sort of issues did you hit? In all honesty I'm not sure I've been more impressed by another AWS service.
It seemed to try to load data into the wrong table on postgres. That was the one that immediately comes to mind. Honestly poor support is what really killed it for us. But we had other technical problems with it.
We burned 3 weeks just trying to get support to provide a sensible response. I never got the sense anyone replying to us knew any more than the surface level about the infrastructure of how DMS worked.
We kept our table mappings pretty much mirrors of the source tables. Any data transformation we managed on the target cluster, not through AWS DMS.
I've used it fairly frequently over a number of years so maybe the issues we hit on the learning curve have dimmed.
We also deliberately kept things as simple as possible at first and focused on DMS's ability to move the data from source to target and really only tweaked settings that increased security or reliability. We stayed well away from any of the data transformation or more complex functionality.
One issue we hit were any schema changes totally messed it up. I don't have my notes in front of me, but we were constantly hitting data that wouldn't migrate, or that things suddenly broke whenever things changed.
Interesting we managed several schema changes when using AWS DMS for replicating data over a long period between MySQL and Postgres clusters.
We treated these carefully and tested as we made them but never had any real issues with them. From memory DMS could cope with adding columns pretty transparently. One setting we invested in configuring and understanding was to allow DMS to attempt to recover from replication failures. This allowed it to error on DDL changes and attempt to recover. This usually involved restarting the task, but it would do this transparently as part of the recovery.
What sort of issues did you hit? In all honesty I'm not sure I've been more impressed by another AWS service.