Just flew Qatar back from Maldives connecting to SAS & UA in Europe. Asked QR if they had an interline aggreement with UA and they said yes. My bags were transferred and arrived in US with no problem. Also, they provided boarding passes for all legs of the journey. All flights were business except the SAS leg, but that only seemed to matter for them to print out the tickets with the correct class noted on the border of the boarding pass. I was impressed at how seamless it was.

I did not ask UA on the outbound if they would interline to QR as I assumed the answer would be no. We had plenty of time to retrieve bags but I will try on my next trip like this in November to see what they say.