The the most common gear style, advantages include a great gear ratio range, with lots of low speed torque as well as top end gears for high speeds. It is the most efficient drivetrain system, so the least energy loss to friction. Bikes with external gears are easily accessed to service, and changing flats on mid drive bikes with derailleurs is pretty straightforward compared to internal gears. There is universal support at almost any bike shop. Disadvantages: for eBikes shifting over the entire range when the bike is at rest it can be a problem - since less active cyclists can forget to down shift when coming to a stop . Wear is going to be higher than internal gears, especially on an ebike when frequently riding is wet dirty conditions. Full chain guards cannot be used so dirt and grease can get on your trousers.
Gear ratios (the difference between the highest gear and the lowest one) is usually 300-400%.