Ok, I did write a longer reply, but something crashed and I lost it.
Conversion kits such as Clay's MW kit run on the original game hardware, with an additional PCB containing extra menu code, bankswitching etc. these are clearly NOT emulation based, although their legality can be questioned. I've taken a look at some of these for misfitMAME in the past, although not had time to finish them.
The Custom Mutligame PCBs from China, Hong Kong, Korea are are almost ALL emulation based. Even if the hardware is NOT PC based. (think about it, how else are you going to run games which were designed for many DIFFERENT platforms on a single platform without having to rewrite the entire games, which would take forever)
I've been sent partial dumps of several of these from fried boards. They contain tell-tale signs of being MAME, for example mame.cfg files, mame.dk readme, MAME romset names, disclaimer strings disabled but not deleted etc.
It comes down to a simple question. If you're making a multi-game PCB and don't care about licensing any of the games, why would you license the emulator? A number of official licensed konami packs on the PS2 are clearly MAME based too, and not even they bothered to license it. They have 2 options A) develop their own emulator, which costs time and money, or B) do the same as they did for the games and just use somethng that already exists, unlicensed.
The choice for them is simple.
Old versions of MAME run fine on old hardware, you could easily run most of the z80 games in a very old build on a 100-150Mhz system. Compiling for a 100-150Mhz PowerPC, ARM, or SH4 CPU is no problem at all, and has already been done for the various ports (GP2X, Digital Camera, Dreamcast etc.)
If you've studied the korean arcade industry at all you'd see that the majority of their products are unlicensed hacks of existing games, it's exactly the same with these multi-game PCBs.
I haven't seen ALL of them obviously, but I can tell you that all the ones I have seen have been MAME, or at the very least very heavily based off MAME. I've heard of a couple running retrocade, but those would probably be PC based ones because retrocade wasn't open source.
Again, I'll stress, if its a conversion of an original game PCB, with the original CPU and original graphics chips then it's not going to be MAME.
If it's a custom PCB with 'modern' hardware running a wide range of games then it's going to be emulation in one form or another, and that form, in all cases I've seen, has been MAME, including some of the ones which only run a limited number of games, which is usually because the hardware is only powerful enough to run them / the MAME version on which the PCB is based didn't really support anything newer. (Take a look at the old Digital Camera ports etc. and see which games ran well on them, you'll find the selection is very similar to those PCBs)
I'm sure we'll have fun trying to emulate these one day, we don't approve of them, but we're meant to document history, and these are now sadly part of history. It will be interesting to try and identify which mame version each is based on, and if any of them were actually original.