Is the MPC era from the early 1970s to the mid 1980s?
Is top of the line MPC very good, but most of it cheap plastic and not so good?
You have the date range pretty much correct: about 1970 - 1986.
Now, I am going to talk about one of the "cheap" sets and show you why MPC detractors deserve nothing but low-quality, high sulpher coal in their stockings.
First, an over view of a set my grandfather gave me in 1972. The first train he gave me was Marx (in 1965); this was my first Lionel. I was 8 years old. It was leftover inventory from the previous year, so far as I can tell, a Double Diesel set, repacked by either Sears or Lionel, without one of the cars and the B unit and a Wheaties boxcar substituted for a bigger boxcar. If Grandpa got a box, I never saw it.
(Sorry about the picture quality: my camera hates looking into a bright light, and I can't extinguish the sun.)
Now, let's look at the engine, a run-of-the-mill 8020 Santa Fe Alco, dime-a-dozen. This engine has I-have-no-idea how many 100s of hours on it. The brushes have been replaced once. Once, when I tossed it into my toybox (yes, I was a kid, and it was a toy), the rear (metal) coupler broke off. A quick trip to the Service Center (remember when MPC actually maintained the Postwar service Center network, which is more than we can say for today's Lionel) set that problem to rights. I also had to buy a replacement horn. There are scratches on the paint and decals, and the luster of the silver suffers from dust. But this engine runs Every. single. time. I put power to it, and it can haul quite a bit more than its own train. I cannot say as much for ANY of my TMCC or Legacy engines.
Next, the first car (this train ALWAYS runs in this order simply because this was the order Grandpa set it up ion the first time I saw it).
The 9140 Burlington Gondola is derived from the bigger of the PW gons and has a sharp green paint scheme. One coupler is a dummy, but the trucks have metal wheels.
The next two cars.
The 9040 Wheaties boxcar is derived from the Scout boxcar and has no working coupler, but it does have metal wheels. Its paint scheme was General Mills tooting its own horn a bit, but it is certainly colorful!
The PC 9300 Log Dump Car still has its original three logs. The mechanism works just fine and has survived unbroken. It has 2 working couplers and metal wheels. Because of the stripped-down nature of the set, I never had the activating track and had to dump the car manually. I do still have the plastic tray into which the logs rolled.
The last two:
Knowledgeable collectors' eyes will see that the GN 9011 hopper is a rare car. About 3 billion of them were made in sky blue, but this much darker, brighter, Royal blue is genuinely rare on this car. Roland Lavoie labels it "Very rare" and notes a $120 (EXC)/$265 (MNT) value (p 243 of the 1970-1991 Motive Power and Rolling Stock guide). It is by far the "cheapest" car, with one dummy/one working couplers and plastic wheels.
Finally, the 9061 Santa Fe caboose; again, one of many, many. I really admired the car when I was a kid because it had end railings and ladders. It has only one coupler (though it it works).
None of these cars has ever needed any repair. The wheels--even the plastic ones--are the fast-angle profile, a significant contribution MPC made to the model RRing world. It came with a blue 4150, 50-watt transformer and a figure eight of track.
Yes, it was cheap. Some plastic wheels, plastic gears on the engine, 2- position e-unit, many dummy couplers, derived from the near-bottom of the PW line. Only one car sparks any interest among collectors. The others are literally common as dirt.
But "not so good"? Oh, my friend: this set is very, VERY good! It has given me untold hours of enjoyment, taken all the abuse I have thrown at it nearly without complaint, and it retains most of its vibrant colorfulness and all of its functionality. No postwar set would have given me any more fun or joy. No modern (Digital Era) set would have been remotely as trouble-free and reliable.
MPC deserves a much better reputation than PW snobs and later-era electronic junkies have given it. Long Live MPC!