So from an emulation standpoint, this core is just as good as the standalone. But there are two glaring ommissions - save states, and an onscreen indicator that the difficulty mode has been switched. A quick check at the github, and I can see someone was complaining about lack of save states (that previously existed) over a year ago. As for the onscreen indicator, once you’ve played an Atari 2600 emulator with that, you just can’t accept anything less.
When you have a real Atari 2600 sitting on the floor, you can see which difficulty settings are selected. With an emulator, you just don’t have that. Standalone Stella compensated for this by giving you an onscreen indication in the form of text at the bottom of the screen, and it was the ideal solution. With RetroArch you really have no clue what is set to what. I mean I suppose you could just reset the core and pay very close attention to what you press, but what if you didn’t hit the button hard enough or something? Plus, it’s nice to have confirmation as to exactly what your settings are.
Stella needs these things. It’s so close to being perfect, yet without these two features, I’d still rather use the standalone. I don’t expect every single feature of every standalone emulator to make it into every core, that’s fine. But this is just one of those cases where it matters.