This functionality is experimental and may be changed or removed completely in a future release. Elastic will take a best effort approach to fix any issues, but experimental features are not subject to the support SLA of official GA features.
There are some limitations to how fields can be rolled up / aggregated. This page highlights the major limitations so that you are aware of them.
The Rollup functionality allows fields to be grouped with the following aggregations:
And the following metrics are allowed to be specified for numeric fields: