If you're sure the mame dol works in wiiflow with the same game you've tested then yes there's an issue with Usbloader's dol loader.
For ScummVM, in WiiFlow this plugin doesn't work and is not implemented like the others. fledge68 should know more about this one. You could try the plugin 15 too : https://gbatemp.net/threads/libgba-gameboy-advance-library.488984/
For ScummVM, in WiiFlow this plugin doesn't work and is not implemented like the others. fledge68 should know more about this one. You could try the plugin 15 too : https://gbatemp.net/threads/libgba-gameboy-advance-library.488984/