Ugh what's this? they can't even emulate Galaga correctly? why not? it's so old
I believe everything in Galaga is done except for the starfield and the discrete sounds.
The starfield isn't done because (IIRC) it's a custom chip with little/no documentation on it.
The sounds haven't been done because no one is working on the discrete sounds for it, that I'm aware of.
Problem is that when the games are in there for a while, no one wants to work on the 'older' stuff. It's not 'fun' anymore. The fun is getting newer games to work. So the older stuff sits there.
There is a new emulator called
Tickle that has a lot of the stuff Mame is lacking emulated in. Read the notes on Galaxian/Galaga.