Symptoms
Assume you have a task to send daily reminders about pending K2 tasks.
Diagnoses
Essentially there are 2 tasks:
1) To pull K2 data about tasks - good task to be implemented within K2
2) Scheduled sending of email on a daily basis
Second task is not a good fit to be implemented within workflow, as it requires endless loop and consequently it is not a good fit for workflow engine when there more conventional task scheduling techniques.
Resolution
Right approach will be to create workflow which generates and send required email. Daily sending of such emails may be implemented with Windows Tasks Scheduler by means of running respective process via PowerShell.