Currently I'm only at around 5500 tracks.  All tracks have working artwork, stored both in metadata and as folder.jpg, usually at 1000x1000.  Thanks to writing this I have just found out I missed an artwork rescale out of my sync settings which explains a small (1-2s) delay displaying some artwork 

Controls:

Artist, Song, Album and more are manually selectable from the left hand side of the screen.  Not been in the car for a few days but I think you click a browse button in bottom left first when on the Media page.  Lists can be scrolled or searched using an on-screen control/keyboard or scrolled using the dial to the right of the screen.  If I want to go back to shuffle all, I just go to Song, picked the first one to play it, make sure shuffle is on and then start skipping.  I haven't tried playlists yet as I haven't found the need.  On the playback screen, there is an in Info button on the right to show more details about current song and a Tracks button which shows all tracks in the current playback queue.

Voice commands work reasonably well for me, better if the windows are closed to minimize background noise, but sometimes you have to trick it.  For example Rammstein might be need to be pronounced Rammsteen.  If it's not 100% sure what you said you get a list of about 6 items to choose from and just have to say the number next to it.

Limitations:

Looking online, there are a number of US Dodge forums that have mentions of a file limit.  They seem to suggest there is a hard limit of around 20,000 files based on some testing for a Dodge Durango with a uConnect system.  Obviously, this is not the exact same setup we have but may give some clues as to what could be happening and some suggestions to play around with to see how file/folder setup affects the system. https://www.dodgedurango.net/threads/2019-uconnect-max-song-limit-over-usb.73566/

The limitation may come from whatever method of indexing the system uses internally.  You can tell its doing something as trying to perform certain actions just after inserting a memory card you'll be told that it is currently rebuilding the media library or something along those lines.  The system has a limited amount of RAM/storage so the more tracks you have, the more it has to store in its database to enable searching, voice features etc and the longer it will take to perform those actions.  This is a presumption, as I imagine it would do something like this to aid performance, same as Plex, Emby/Jellyfin, Kodi etc..  With Blue&Me on an early car, the suggested storage limit was restrictive, possibly for the same reason.  At some point the Blue&Me units were updated and worked a bit better with larger storage (for the time).  I still used to hit a problem where there were noticable audio stutters above a certain number of tracks, which I presume was due to a memory limitation.  As a result, maybe they have implemented a hard limit in uConnect to avoid issues. 

FAT32 itself has some limitations. Your unlikely to hit the file size limit or directory depth (Drive:\Folder\Folder\Folder\Folder\.. and so on) with an audio library, but there is also a limit to the number of files per directory.  Without going into the specifics and to put it simply, the longer the filenames, the less files there can be in a directory but the number is still up to 65k.  I have it set up so everything goes into folders eg USB-Drive:\Artist\Album\01 Artist - Track Name.m4a that way I'm never going to get close to it.  I may have to change this method if I hit issues and the testing on the Dodge forum has some suggestions.

The manual wasn't very clear on what is/isn't supported. When I first picked up the car I played around a bit to see what worked, sticking a few (3 albums or so) on to a card using different formats/filesystems.  I tested exFAT, and I believe NTFS, which from memory both worked.  For audio, I usually use m4a format, unless I only have an mp3 for a specific album (Amazon purchase for example).  I also tested a few FLAC files just after I collected the car, but I can't remember if metadata/album art worked correctly but they did play fine.  I have a master library saved in lossless formats that is used for a Hi-Res portable audio player so I may switch over to this in the car at some point should storage space not be an issue.

Compared to the Blue&Me systems used in pre-facelift Abarth and Alfas of the same era, the new system is a lot better.

Can't remember off hand when Android Auto/Carplay started to be included as standard on some models.  Before that it was a £150 option I believe.

The only thing that puts me off AA/Carplay is the extra wear to the mobile battery due to the increased charge cycles.  I have used it briefly and is a good standby option for when Nav stops getting map updates but I prefer to stick to the USB option for now.  I use an app called TuneFUSION (by the people behind dBpoweramp) for some devices to automatically modify the metadata/transcode formats to meet the players requirements but it also allows me to select albums/artists not to include as part of the sync.  Don't want Christmas music on the USB stick all year round.

If you can access your media at home securely from your phone you could use your media servers audio app, something like PlexAmp for Plex or whatever Emby/Jellyfin use as there app.  Just need a decent signal and a lot of data and you could stream however many songs you have at home.