Here's what worked for me:
If rpcs3 is loading with the games list every time you launch a game or throwing an error message:
* First off, understand that RPCS3 roms are in the RPCS3 folder itself under LaunchBox\Emulators\RPCS3\dev_hdd0\game (thats where they were for me)
* Do not go into the Emulator settings in LB and remove --no-gui (despite what AI may tell you). That command line is actually necessary to boot directly to full screen mode and still works with new versions of RPCS3. There's no option to turn gui off in the emulator's config settings so you have to use this command line in the Emulator settings.
* The fix for me was to make sure the path to the rom directory is the same as the one that RPCS3 is showing in the game info. Launchbox was importing the wrong file pathways by default that seemed to work in old versions of RPCS3.
* For example, Stick of Truth imports to launchbox in directory: LaunchBox\Emulators\RPCS3\dev_hdd0\game\BLUS300000\USRDIR\EBOOT.BIN (it may be a diff BLUS folder number, I forget the exact pathway) but the problem is that launchbox is pointing it to a BLUS folder instead of an NPUB folder)
* If you open RPCS3, scroll to Stick of Truth, right click, and look at game info, you'll notice RPCS3 imported it to the proper directory of LaunchBox\Emulators\RPCS3\dev_hdd0\game\NPUB31306\USRDIR\EBOOT.BIN
* To fix it, change your launchbox rom lauch directory to LaunchBox\Emulators\RPCS3\dev_hdd0\game\NPUB31306\USRDIR\EBOOT.BIN and it should work. Do this for any other game that is using the BLU folder instead of NPUB and confirm which NPUB by matching it to the RPCS3 game directory. Do that while keeping the --no-gui command on and it will boot in full screen with no games list as it should.
Hope this helps.