In Premiere Beta 26.0.0 build 27 (macOS), I found 2 bugs:
1) Can't use lib:AdobeXMPScript as ExternalObject
new ExternalObject("lib:AdobeXMPScript") // IOError: I/O error
2) Can't use alert function
alert("Hello") // alert is not a function
This breaks half of my extensions. I rely on AdobeXMPScript to add metadata to project items.