Compatibility is 99% that of the original VBA, give or take.
We've broken some games that worked in the original, as well as fixed some issues that caused games to fail. More or less we've made a trade off.
Some games will have issues regardless as they did on the GBA (Mode7 FF games like 4 and 6).
At some point, we will be tearing the emulator to pieces, and performing a full rewrite of the main, gfx and sfx cores. Some bugs can only be fixed this way.