I've been told that PowerApps can't access Sharepoint behind ADFS, as per: https://powerusers.microsoft.com/t5/PowerApps-Forum/Access-Denied-to-Sharepoint-2016-On-Prem/m-p/263...
Given Sharepoint and AD Federation Services are Microsoft flagship products and PowerApps is the up-and-comer, it would be appropriate for the latter to support the former.