My '21 Laramie plays okay from both front USB media ports - my wife uses USB 1, while I use USB 2. Between our flash drives, there are 1000s of songs - around 1,500 on hers, not quite 3,000 on mine. They are a combination of mostly MP3, but there are some M4A purchased thru iTunes. They all must be unencrypted - in other words ripped from CD or purchase downloaded thru a store like Amazon music (or the many other services that sell MP3 formatted files). If it's subscription music, or something copied over from streaming, they are likely to be encrypted.
My issue with playing large catalogs centers around the shuffle algorithms, which don't handle large catalogs well. I find they select a pool of 200-300 songs, and then shuffle between that. To fix that, I've turned off Shuffle, and then turned it back on so it selects a new pool. It's the same thing on my car (Honda), only Uconnect takes longer... But without shuffle on, I can navigate thru the entire catalog, which would almost be worse than texting if I did it while I was moving.
Another thing to consider, is Uconnect makes extensive use of the metadata within the MP3 files. If your files are ripped from CD, and haven't been "massaged" by a media application like iTunes or Windows Media Player, then that metadata is likely missing. I don't know how Uconnect reacts to that.
Main thing I wish they'd change is allow the restart of the last song playing to be optional...