Dynamics CRM 2015 release brings a rich tool for system customizers to be able to do their work with less dependency with code. The most useful features in Dynamics CRM 2015 is the Calculated and Rollup Fields.

First of all there are some good post from fellow MVPs regarding these new features:

Calculated Fields (by Leon Tribe): http://blogs.msdn.com/b/mvpawardprogram/archive/2014/12/15/dynamics-crm-2015-calculated-fields.aspx

Rollup Fields (by Jukka Niiranen): http://niiranen.eu/crm/2014/12/analyzing-customer-behavior-data-crm-2015-rollup-fields/

So, I did my hands on with these new features to test and found out that some limitations in regards these new features:

1. Currently there is no way to upgrade/update existing fields (Simple) to calculated or rollup fields. This situation could be common when there are some fields that calculated through other methods before (plugin/WF/js) that would like to be updated with the new functionality.
2. The calculated or rollup fields need to be created first in metadata before it could be configured. Therefore if the configuration process was not completed during the creation of the fields there is potential “orphan” calculated/rollup fields that do nothing.
3. The rollup fields are calculated asynchronously through the system job, therefore there will be some delay to get these fields to be calculated.

I hope this helps!