Ask those of us who have routinely purchased on UA.bomb because the codeshares were cheaper than booking US. And this happens frequently.

In this case, while per the US website you're not supposed to get US credit if flying a codeshare / ticketed by another carrier, you do. And with US, you get 100% credit for every revenue fare class in Y on UA (and CO too).

You deal with UA for pre-departure ticket changes and US for irrops. But you can still get your preferred upgrades on US, you can still use the move-up program, you saved money. But you had to use .bomb which is debatable whether it's better or worse than the current incarnation of the US website at times.