I would agree that the ability to maintain/replace/re-build batteries is one of the biggest factors.
I bought a BionX D-500 retrofit kit in 2016 and installed it on my 27 year old Bruce Gordon Rock & Road. After the BionX bankruptcy I began collecting spare parts. I now have a second wheel/motor, spare console & charger plus three additional batteries. I'm sending the oldest battery out for a re-build soon. 16,800 miles on the motor and still running like new on a bike approaching middle age!