It's more than just USB, but it's the first that comes to mind. Label LIVE has built-in drivers that talk directly to USB hardware (thermal label printers). The actual "label printer driver" part (talking ZPL, etc) is cross-platform, but the middleware of handling the USB stack has many edge cases and hugely divergent implementations that wounded me greatly (since recovered, thanks!). On top of USB you have "system printer" queue support, local font support, fingerprinting the system for licensing, lib FTDI for weight scale integration, fabric JS w/ all the hidden beasts of WebGL, WASM builds for higher performance dithering, different sandbox requirements, app signing, app packaging... and those are just off the top of my head. There are probably a dozen more things I'm forgetting that would just wreck me if I wasn't mentally prepared enough.