IME, commuting with folding bikes in NYC metro since 2013, hydraulic brake on these small wheels/narrow tires are just overkill, if not hazardous in loose/wet surfaces.
Smaller wheels/tires with smaller contact patches, not ideal for higher speed and/or heavy rider/cargo load; mechanical disc brakes should be plenty powerful for stopping power.
Personally, I prefer hidden battery in the frame or seatpost, stand alone battery that protrude from frame, not so attractive.