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![](https://beehaw.org/pictrs/image/1801fd1d-aa94-416b-ab31-be0103b4ccfa.png)
Iirc the biggest reason they can be dangerous is that the dropouts (where the rear wheel mounts) were never designed to stand up to the torque of a hub motor. You can brace it with extra metal or stick with a low wattage motor and make them pretty safe.
My local shop lists all their prices, here’s what they have: