Another reason: Your tasks/dags aren't portable from one system to another. Your tasks+dags are your code base. You can't create something and share it with others very easily.
This is why I wish attempts at standardizing ways to express DAGs in YAML/JSON/XML like CWL [0] and WDL [1] had more steam to them. If these standards took off, you'd be able to take your workflow and execute it on another batch system scheduler if you got tired of your current workflow orchestration tool.
[0] https://www.commonwl.org/ [1] https://software.broadinstitute.org/wdl/