We use subtasks a lot in my team, they really help manage a process. But we’ve found they cause issue, particularly under recurring tasks, when users get lazy and don’t mark the subtask as done before they mark the main task as done. We end up with “orphaned” subtasks floating around in the list of incomplete tasks.
The other issue is that allowing users to mark the main task as done before the subtasks are complete makes tracking a full project difficult. For example, if raising an invoice for a job is one subtask but it’s not marked as complete, it’s not obvious that a potentially crucial step in the process has been left out.
I haven’t seen a way to do this currently, so perhaps you can consider a future feature where we can block users from marking a task as complete until every subtask is marked complete. Personally, this would save me a huge amount of frustration at users who are lazy and less interested in doing things “properly”.