FRLG TASes have synced between console and emulator with respect to frame timing. This indicates that mGBA is likely accurate enough that with the FPS set to Native it would be indistinguishable from console.

The reason that I haven't advocated to have it fully accepted in the same way as gen 1+2 is that the sync broke down at the first battle in FRLG because some hardware timers are not emulated perfectly and though the frame count was the same, the RNG (specifically the TID which seeds RNG) was different to those same frames. So it's possible that a game like Ruby that doesn't rely on those timers is nearly as accurate on mGBA as gen 1+2 on GSR, but that also hasn't been tested completely because bizhawk doesn't have mGBA's dry battery option yet.