FBNeo generally handles or Non-Merged sets well. If you are using a Split set intended for PC MAME, you might be missing the "Parent" ROMs required to identify the game, leading to the "Unknown" error.
FBNeo updates weekly. With each update, romsets change:
archives. If a single file is missing or named differently than the database expects, the whole set is flagged as unknown. Non-Merged vs. Split Sets: