About a year ago i searched for similar answers as we use Alfresco in our project. As far as i can see the provider switch is nothing more then a tool to help configuring the server uri. Perhaps it is also used as a switch inside the pega connector library to for vendor specific interfacecapabilities but i doubt that.
Regardless of the provider chosen, the CMIS interface is build on the Apache chemistry opencmis library which is available in the pega platform.

It seems basic cmis functionality for creating en removing documents from the root folder is implemented in the OOTB cmis connector as well as fetching and updating documentproperties which do not need vendor specific api calls.

In our project we needed additional functionality like creating case specific folders and moving documents between folders. This functionality was not supported (and still is not?) by the OOTB Pega connector. We decided to create our own activities calling the apache opencmis java library from java steps to get what we needed.
 

There is no documentation provided by Pega on using any specific CMIS provider.
You should view the documentation from IBM on exposing its CMIS capabilities to CMIS client applications including Pega.

The "Experimental" label means that Pega has not tested interoperability between that version of the Pega platform and the CMIS provider. As always, testing should be done on your side before use in production.