This bike booster will probably come in at less than half the cost but the Bimotal kit is another option that doesn't have the tire wear issue, offers better performance, and the form factor is much nicer.

https://bimotal.com/products/elevate

Also:

* you cannot install them on bikes with a rear rack. Here most city bikes have them.

And, don't forget: easy to steal off even a well-secured bicycle.

Do they work with mountain bikes? As stupid as this sounds I like mountain biking but I don’t have stamina to climb up hills and then keep doing for a long time. I have an electric bike but designed for road

Wet-weather performance in particular seems like the thing I'd want to see independently tested

Yeah. I don’t see weather or theft addressed in this design.

I recall a wheel-mounted rotor system from a few years back that did address theft, tire-wear, and weather, but it suffered from far too little torque and very limited battery life.

Personally I also don't see the niche they serve. If I'm going far away I take a train or a car. I pretty much exclusively use my bike to ride to a nearby grocery store or do other close-by errands. If I'm doing short errands like this, I really prize being able to get on and off quickly and lock my bike up quickly. A device like this needs to be taken with you when you leave your bike to prevent theft. It's a whole nother step and a lot of extra weight

Looks like you can't have a passenger anymore either. I like to have my kid on the backseat of my e-bike.

I'm not in the market for this, but I am in the market for making my cargo bike electric. Having to slow down at a traffic light and then start from zero absolutely sucks. It goes very slow, it is rough, and you won't have time. I'd use it just for that specific use-case.

I made one in 1985 for my 10 speed bike, it was pretty fun for a while. I was in my early teens at the time.

Are there any mechanically coupled solutions that can be field installed by the owner?

I had a gas powered goped that worked that way years ago. Had all those problems but it was fun. The second you hit any water the drive shaft would just spin against the tire.

I remember back in the 90s some guys at my school fitted a whipper snipper motor to a 12” kiddy bike with a friction drive (I think it was a skateboard wheel fitted to the output shaft).

They could have made basically the same thing except with a sprocket to help drive the chain.

Tires wear out super fast. It's hard to say there is a market for this other than the uninformed.