eBikes have evolved since the second iterations that were direct drive rear motors. Direct drive rear hub designs are still relevant for "unlimited" eBikes that are not street legal. Also a staple of bargain online sellers - you can't feel just how heavy they are on the internet. Suited for more of a moped experience. Still offered on premium brands such as Stromer, but most quality eBike shops have focused on sporty mid drive eBikes because reliability and manufacturer support is far superior. These dated designs are kind of like a "gateway drug" to more modern mid drive options that will retain their value for many years to come.
Improving on the front hub design, with more powerful motors and pedal activation of assistance (when pedal are turning) - you could ape up a hill like never before. Usually featuring heavy "direct drive" motors (essentially a massive set of magnets) that delivered substantial acceleration. You could select modes to dial in your power and normal bikes could be retrofitted.
Jerky - a pedal rotation (aka cadence) sensor is like a throttle operated by your feet, but one that only knows all on or all off.
Heavy - to give you enough torque, these direct drive motors have to be as big as possible, consequently adding 13+ lbs, + the battery.
Efficiency - direct drive motors are about 30% less efficient than mid drives - the giddyup kick from a dead stop is what really eats into efficiency - consequently huge heavy batteries had to be used.
Gorilla - retrofitting tears up a bike that was not designed for this kind of tank-like weight and power. Very ropey looking as well.
Batteries - 1st Gen lithium-ion had better energy density, but for example, LiFePO (1rst Gen Lithium-ion) voltage would drop suddenly, making range calculation hard. Form factor was not standardized nor were safety certifications that were subsequently adopted.
Handling - these brutes had their outsized weight distributed all wrong for a normal bike riding feel. There was a gyroscopic effect (making turns feel weird) with the unsprung wheel weight. All that weight towards the back made the bike want to wheelie up on steep climbs. Having so much more weight meant compensating with heavier suspension or having a very harsh ride.