I built up one 7 years ago with a Golden Motor and a 48v/11Ah Duct Tape battery on a Yak. Used it primarily on my Townie. Carried the battery on the trailer along with a conveniently sized cooler to do chores and get groceries. It was powerful enough to get me up the steepest streets in town which are pretty steep there actually. Took a few 25 mile jaunts with it but it cut out at 20mph and had a lot of parasitic drag trying to go any faster unless you were coming down the aforementioned hills.
Never had any handling issues but was sensitive to giving it much "gas" when doing a sharp turn. It helps to have some weight aboard like the battery as it can tend to be a little light traction wise without any.
It is a great concept that never has gotten there yet, if it ever will. But the ability to have an e assist that you can ditch easily or swap to other bikes in the fleet is definitely valid. Ride to the trailhead, stash it, ride, then hook back up and pick up the groceries on the way home.