Good question! As far as I know a mid drive ebike kit requires you to sacrifice your front derailleur and 2 or 3 chainrings. So your 27 speed mountain bike now becomes a 9 speed with one front chainring . The only thing you can choose is the size of the chainring that comes with the mid drive kit. If you want to go fast you get a large chainring and if you want to go up hills you get a small one. It seems you can't have both.
If anyone knows about anything different, please feel free to reply here!