The Swytch 25 miles and 50 mile ratings don't exactly compare to other kits mileage assumptions. I put a Leeds 250 watt front wheel kit on a bike (not folding) for my wife over a year ago. It works great. Leeds is a simple concept - you just press a switch when you want some electric boost - otherwise just pedal. The hub motor doesn't appear to cause significant drag when just pedaling.

Leeds mileage ratings actually assume your cover the miles under electric power. The Swytch mileage number must presume you are pedaling the whole way also (the battery amp hour numbers are too small otherwise.) The Swytch 25 mile battery is 5 AH. The Leeds 24 mile battery is 12.8 AH. Of course the Swytch is pedal-assist.
The Swytch pricing is attractive. I'd love to see a real world review of a Swytch compared to a unit like the Leeds.