It's not bad, but there's two major features I wish were implemented in Genesis emulators.
First is start the game in full screen as an option. The reason I like Mesen's way of handling it is because I have the emulator launch in full screen and can scroll my mouse to get to the menu. SNES9x is easy too since hitting escape toggles the menu bar.
The second is having a better way to switch controllers from 3 button to 6 button using the same controller. This one might require a bit more coding to implement properly. But modern controllers all use the Xbox/Playstation/Switch Pro layout with four face buttons and 4 shoulder buttons. The input mapping 1 and 2 on JGenesis seemed to be confused as to what I was doing. For Street Fighter and Mortal Kombat, I'd want the controls set for 6 button and for a game like Streets of Rage I like having (SNES naming) Y as special, B attack, A jump. Having to configure that every time is a chore.