I have had the following assist systems:
Big and easy to see
Will haul a lot of cargo
Electric and peddle
Advantage is endless range and lots of power
The disadvantage is noise, street legal only (US - not for bike trails.
Direct belt drive - no gearing leverage.
Observation: I had it installed on a two wheel townie and because it was a step through I ended up having to buy a longer throttle cable - so if you go this way make sure you specify a longer cable.
Easy and inexpensive to self-install on your existing bicycle
Direct drive - no gearing advantage so if you have a big hill you may have to get off and push it up
Easy to swap out the front wheel.
No throttle, just a switch, either on or off a bit uncomfortable.
Would not make it up the hill I needed to go up.
Would not recommend this one.
Used lead acid batteries
Not a bike conversion so no local support or parts
Rusted in one year