No DS slot cart plays GBA ROMs" was once the statement that echoed around these forums on a fairly regular basis. However that was before enhanced flash carts turned up and when the dust settled there it was the DSTwo that came out on top. It had a GBA emulator called NDSGBA and though it was far better than many would have hoped the GBA emulation on the PC and other devices had been near flawless for years and GBA flash carts were equally high performing. You could still play a lot of the spectacular GBA library but there were notable exceptions and drawbacks.
However following a little bit of community action the sources for NDSGBA were released and the TempGBA project was kicked off in earnest under the care of Nebuleon. Yesterday saw a new release that is something of a milestone with some nice speed improvements all on top of the massively enhanced GUI that the project got early on. https://github.com/Nebuleon/TempGBA