They have zero A330 pilots, it adds a new fleet type which makes them have to do a lot of work on the maintenance end. A330 simulators will need to be ordered and there will need to be new fleet type training for pilots and FAs. Why in the world would any airline, no matter the deal want to have the A338? It's currently orphan aircraft that will have poor leasing terms and resale value. Its either 787 or A321 for this job and I think the 787 has it just like the AA deal.
United is to take delivery of the A350. The A330 and the A350 share a common type rating, meaning creating a larger pilot pool makes United more flexible, so pilot-wise we are not talking about a small subtype.
Also, transition from an Airbus narrowbody to a widebody is quite easy. The A330neo also has the same cabin outfitting options as the A350 (depending on customer wishes ofcourse), so cabin crew can also work on both types. A double simulator (for A330/A350) is also available on the market.
So that leaves maintenance. Not a small issue, but all the other items could be managed quite easily.