What are the best practices for using client tasks? How can I help my clients understand and gain comfort with that form of engagement?

The Karbon support site has a whole section regarding client tasks - https://help.karbonhq.com/en/collections/102205-client-tasks

Within the guides area, there's a video and an email you can use to share with your clients to introduce them to client tasks. I recommend getting comfortable with client tasks and their reminders first prior to having them auto-sending at the beginning of work items. I hope this helps :)