Every UACO ticket I've had in the last 9 months or so that has been ticketed or later changed to mixed metal (voluntarily or not) has experienced 2-4 of the following issues:
1) Out of sync notice on website and/or inconsistent flights/seats on UA.bomb and CO.sux
2) Unable to assign seats
3) Unable to OLCI
4) Unable to checkin until I reach the airport where the flight originates
5) Unable to print BP until I reach the airport where the flight originates
6) Ineligible for EUA

Phone agents (repeated attempts with both sides) have been unable to ever resolve any of these issues. But an airport agent on the day of travel at the airport where the flight departs always can.

I hate to book mixed metal, but when it's hundreds of dollars cheaper I resign myself to the eventual inconvenience. Other times I'm forced into it due to airline schedule changes and route swaps.