eTap / Di2 cassette shifting should be fine afaik. But Alfine Di2 is another story. After a close look I have my doubts about Alfine Di2 on a Birdy. I would not buy unless I could find accurate dimensions of the motor unit. It might not fit under the curve of the Birdy chainstay/dropout, see photo. The motor unit seems to have a bigger diameter than the hub flanges, almost as far out as the locknut. That would be a problem.


Alfine Di2, note large motor unit


Alfine 11 / Birdy closeup, rear


Alfine 11 / Birdy closeup, side

My "arm pulling patent" isn't quite as I remembered it. The bolt goes in from the outside, and I use a flanged nut to pull on the arm.