UA gets a cut and you'll get potential headaches.

Codeshare passengers often are not able to get assigned seats because the ticketing carrier can not see the operating carrier's seat map and the operating carrier does not want to touch a codeshare reservation until check-in.

If there is a delay, the operating carrier might refer you back to the ticketing carrier because the ticket was issued by the ticketing carrier's fare and the ticketing carrier will refer you to the operating carrier because it is their delay.