Here is the truth on this one because I actually know the answer
Originally the legacy section was to hold no roms, it still does not - just layouts. However Chris updated MFME to 'call home' so to speak to download the roms required from here somewhere in the backend. This of course required me to send all the new roms to Reg to upload and is something I've not done for over a year.
What makes some layouts work and some not. Basically the legacy layouts are factory reset and uniformed. So for example if you have Andy Capp layout 1 running acap1.bin but then Andy Capp layout 4 the designer decided to call the roms andycapp1.bin then I will have changed it back to the original to make the sets more uniform and to cut down on duplicate roms on the backend.
The main culprits in this are the ones in question. A lot of Reflex roms are renamed to get rid of the silly long not needed names. So in short MFME is looking for something that is not in the backend so will report an error. However as folks have said this mainly effects newer games so just get the roms via the normal method.
I've had a hell of a 2021 with illness, losing my main job, starting my own business, moving shops, more health issues etc etc and yesterday just got the legacy section back on track. My next mission once I speak to @Reg is to sort the Rom backend out. This will take time and at the moment maybe 10% of stuff does not work but that 10% are newer layouts and cane be obtained fully working via the normal downloads section.
I will get this sorted, it's just going to take a little more time. Thanks.