I've heard mid drives tend to be weaker as they are directly linked to the cranks so pull on the chain rather than direct drive as the power is right at the wheel . Also front drive being weaker than rear drive . This is why I got rear drive (on both bikes) . The only downside is non-mid drive set ups tend to have a lag in motor response with pedal assist and I can tell you this is true . 6-8 seconds before it kicks in (although my heavily modified  MTB Ebike takes about 3 seconds) . Pain in the a** especially if you stop on a hill . What I do is hit the throttle and pedal at the same time as to not put strain on the motor and not consume so much power then once I get momentum I let go of the throttle and wait for the sensor to pick up . Not sure if this could damage the motor though . Will need to do some research .
 

I have a switch for pedelec vs throttle. I tend to use the throttle in town and the pedelec just as a cruise control. It kicks in after about half a turn of the crank. This is with a BPM rear geared hub.

The big advantage of mid drive is that the torque available depends on the rear gear chosen. This makes them great for climbing hills as well as for top speed. I'm not hearing of problems with reliability of chains or rear gears. One possible catch is trying to change gear under power. ev3em sell a button kill switch so you can momentarily kill the power while changing gear.