The new custom fields that are linked to the task type are not fully implemented yet. Formula task type custom fields cannot be shown on dashboard cards. This limits the usability of those custom fields a lot.

I hope "check back soon" means it is currently being implemented.

Example of a use case: For project milestones we have a custom field "Delay". It calculates the delay of the milestone compared to its original due date. It is a formula field that is linked to the task type milestone. On the dashboard I would like to show a monthly average of the delay of the milestone. This allows us to see if we are getting better in keeping milestone deadlines.