Nebula is by no means fake, but unless your pc is really really really old, imho it's definately a waste of time. Here's the thing about non-mame arcade emulators. They are not as forgiving as mame is about running them in a mame machine.
So, in order to put another arcade emu on your cab, it should probably fill one or more of the following criteria.
1. It emulates games not yet emulated by mame. The only one's that come to mind atm are the model 2 emulator and viva nono.
2. It emulates already emulated games, but the performance is much greater. Zinc and the u64emu would fit into this category, however at this point, ki and ki2 will run on a modern pc under mame, so there isn't much point in running it. For those of you thinking, kawaks and nebula, think again. Those games run full speed in mame on a puny 450 mhz, so if you are worrying about that then you have other concerns. The kawaks, nebula interfaces are too cumbersome and unfriendly for cab use just to play games you can already play on an emulator that is friendly.
3. The emulator actually has games worth playing and the games are actually emulated to the point that they are playable and useable on a mame cab. This throws out a LOT. Imho viva nono is too glitchy to be useful and the games aren't good enough to override it's problems.
So basically.... zinc and model 2 as far as arcade emus go. Yup... that's pretty much it.