I just ran a Lightning Experience Readiness Check on my org.  And "Salesforce to Salesforce" was not flagged as "unsupported" or even mentioned in the Readiness Report results.  But in Lightning UX I am completely unable to access any aspect of the "Salesforce to Salesforce" setup and configuration.  

It was only after searching the Online Help Documentation that I discovered that "Salesforce to Salesforce" is only available in Classic UX.  The fact that this isn't even caught or mentioned in the Readiness Report results is a bit, ummm...  disconcerting.  

Does the Lightning Experience Readiness Check even know what features don't work in Lightning?  Or are we just supposed to turn on Lightning and find out "the hard way" what does (and doesn't) work?