The advantage of such a conversion seems to be the easy swap ability. If/when I go powered*, that isn't a feature that interests me. I'll either buy a purpose made E-Bike or do a mid motor conversion.
*I love cycling, just being out on the bike, it's my major source of income, my primary transport, my leisure activity and my way of keeping reasonably fit. A chunk of the pleasure comes from the physical exertion, so I won't choose to give that up. But should there come a time when that no longer gives me pleasure or I'm not capable of it, I'm not going to deprive myself of all the other benefits.