Interesting. I was getting a black screen with all the games I tried. I could access the RetroArch menu and try to load a new rom and I will not see any error messages, just a black screen and no sound.
The roms/bios I was using I extracted from an RecalBox 6.0 image released by Warpgate. I like the game selection but I don't like the heavy modding done to the theme.
I ended up deleting the neogeo.zip (1.4Mb) file from roms/neogeo and copied a new one under bios/fbneo
The new one is 1.7Mb and can be found as part of a "RetroArch Full Bios Set" xxxxxxxxxxxxxxxx if anyone needs it.
After doing that I tried about 10 games (including Metal Slug 1 through 5 and X) and they are working great. I can confirm they are launching in FBNeo and was able to access the soft dip menu to enable free play, blood, etc.