If you ask me plastic has no place in the motor, frames and wheels.
It just cuts cost for them to make it. I would say the same for plastic body's.
But I have some strong running Lionel engines that are plastic. (body's that is)
So as far as the body's that just a personal choice.
But give me cast or brass any day. (rhymes?)
Now.......,that's just my feelings for O gauge.
Not all gauges.
The plastic motors belonged to some Scout Loco's. The frames were plastic. The 1130 and 2034 look the same but have better motors for the era. One example
One disadvantage is that parts needed to fix one up, are worth more than the engine. More than likely you will end up with one if you buy used trains by the box. Be aware, some cars for these sets had their own type of coupler.