A couple of potential challenges, depending on the fare class booked:

If your UA fare class for the AC-operated segment happens to map over to a "Tango" fare on Air Canada, AC won't provide complimentary seat selection. (Tango is still Tango, no matter where you buy it.) The kicker: you're on a UA ticket, but AC has no possible way of charging for seat selection unless you're ticketed on an AC ticket. Without an AC ticket to allocate their revenue to, they have no way to take your money ... even if you're happy to give it to them.

The response from AC in these situations: You can't do advance seat selection. Period. If you booked a codeshare flight operated by AC, and it ends up being a Tango fare in AC's system, you cannot do seat selection until check-in opens at 24 hrs prior. And AC can't/won't take your money for paid seat selection.

One of the big, ongoing issues when booking an AC flight through UA is that that nobody at UA has any clue what "Tango" is, what their United fare classes will map to on AC, and what you get/don't get on AC. (I've spent a lot of time speaking to reps in some overseas UA call center on this issue, and they have absolutely no clue about AC's fare classes or seat selection.)

Similarly, if you're ticketed on AC metal with a UA flight number (and on UA ticket stock) AC has no way to charge you if you want to purchase a Preferred seat. Even if you want to buy up to more legroom, AC has no possible way of taking your money.

It's one of those often-discussed AC IT fails ... and one that annoys a lot of customers, and prevents AC from bringing in revenue.