Are you talking about Android M and an SD Card?
The reason Google doesn't want people using SD cards is because SD Cards suck. People buy cheap class 2 SD Cards, install their launcher on it, and then rant about Android being slow. But even if you get the fastest, highest quality SD card available, it's still drastically slower than the internal NAND used by modern phones. This is also why you can't stick one into an iPhone. If they were just as good as the internal storage, Apple would just make a special SD Card and charge $2/GB
I don't get why people want to install apps on their SD card anyway.