Custom field support is in a "prototype" stage at this point. There are a number of issues with the implementation such that we ultimately decided it was best to hold off exposing to folks unless they were ok with / aware of these issues. The ones I'm aware of are:

a. Radio Buttons and Select fields result appear as value (not labels) in report results.
b. Multiselect field results include separator character and values (not labels).
c. If field labels are the same for multiple fields, then first field is hidden.
d. Group by with custom data fields gives duplicate entries in report results (summary reports).
e. Searchable custom fields are not available as filters.

The last 2 items are the most significant - but the others are "messy" as well. I don't have a timeframe on getting custom field support up to speed - might depend on sponsorship and / or contributed work from folks outside the core team who need this.