I did nearly buy a Catalina Capri 16.5, which is about the same size as the wayfarer, with one installed. The seller said it would do 5 mph. When asked about the battery life, it was 1/2 to 1 hour – depending on the battery. So you have 5 miles range basically, unless you have a spare battery.
It seems to me that the main downside would be that once the battery is flat, it’s flat. No easy way to refuel, until you return to port.
My Tohatsu 3.5 will run for 50 mins on a litre, so a one gallon tank of petrol will give me 3-4 hours cruising at 5.8 knots. The boat will do 5 knots on half throttle giving more like 6-8 hours cruising on a gallon. A 2 gallon container gives some serious range – about 70 miles.
Being a modern 4 stroke it runs vastly more cleanly than a 2 stroke. I think emissions are about 10% of those from a 2 stroke.
I guess it depends what you want if for.