Is there any feedback from Tableau in this area?
Basically, the answer to your question is no, event based subscription (like data driven subscription in Reporting Services) in Tableau server is not possible.
However if you need event based you need to code it on your own. It can be done with some script-development and Tableau server.
Or you could live with the standard subscription and make sure that the e-mailed dashboards only show information if the user need to take action. You still get the mail, but it will be easy to take action if e.g. your KPIs are over a certain treshold.