ChannelMix OneView separates campaign performance and conversion data for multiple reasons.
Conversions are not attributed directly to performance data
This means that a group of impressions in your dataset is not directly attributed to a specific count of conversions. These two metrics are separated in source systems and are only shown in aggregate. This is the same method that your ChannelMix Data Warehouse uses by separating the datasets and allowing you to dynamically build a custom report that uses the fields you need.
When new custom conversions are created, they need to be available in your dataset
By creating a data structure that expands down (with more rows) ChannelMix can easily support new conversions as they are added to your source systems. If additional columns are requested to support custom conversions, this would result in custom changes and additional time to rebuild the OneView.
Custom conversions are unique to accounts
There is a great deal of efficiency gained when your agency implements a standardized data strategy across all clients. Separating performance and conversion OneView datasets allow your clients to implement a similar data strategy which leads to less confusion across your agency when switching accounts and also sets up the agency to build agency-level datasets to understand what is working at a macro level.
For these reasons, ChannelMix does not support OneViews that include custom conversions as columns.