Just for the record, I've been doing this on more than 75% of my flights for more than 10 years, and never once had even the slightest problem with UA or any other *A carrier before 3/3. I've only had 1 single problem with this before 3/3, in 10 years and that was a single flight originating in Kiev on Aerosvit connecting to UA through FRA. Since 3/3, every time I do this with UA, it's a struggle. The issue is SHARES. Even for a well trained agent sCO agent, it seems to take 20 minutes. For a new agent or an sUA agent, it can take 30-40 minutes. If UA stops interlining bags for itineraries that aren't on the same record, it will be a huge issue for me as I'll need to change connection times from 45 minutes to 2-3 hours to leave the secure area, claim bags, re-check them, then go through security again. I hope this doesn't happen. It's really true that DL won't interline bags unless all segments are on the same record?
The problem with putting all segments on same record is that you have much higher change fees and the change process becomes a lot harder, plus, every time I look at this, the fares become a lot higher and the overall cost of the trip can be as much as 100% higher than with separate tickets - particularly when combining multiple airlines.