I have never had any issues with online checkin and boarding pass printing with United, Air Canada, or Lufthansa, for multiple trips from the US to Europe. This might be because I joined their frequent flyer programs and provided my passport info as part of my online profile for each airline giving them plenty of time to check me out. Every international flight I was still asked to show my passport at the boarding gate for a thorough review before being let onto the plane.
Only on return back to the US have I found difficulties in printing the boarding passes. When I do get to print them, I am still required to stop at the airline's counter in the airport I am leaving Europe from and they replace the preprinted passes with their own.