That's not actually correct. MAME intends to be as accurate as possible (we emulate the actual microcontrollers running the actual firmware inside the keyboard

That's not actually correct. MAME intends to be as accurate as possible (we emulate the actual microcontrollers running the actual firmware inside the keyboards for PC, AT, and PS/2 keyboards as one example) but it's not always there. Sega Genesis is a poor example; MAME runs the entire commercially released library fine, but some homebrew stuff that seeks to stretch or break the hardware doesn't do well (notably the Titan demos). On the other hand, MAME's 8-bit Apple II emulation is cycle-accurate and you can raster-split mid-scanline by counting cycles (as some people crazier than me have done). So it really depends on the specific driver.