I think that those that regard the MPC era with disdain would point to examples of where parts of the product lines were cheapened further after the postwar period.
Some would point out to MPC further removing details from the F3s and the earlier powered units having only a single motor (this actually started in the postwar period). Others would point out how the columns of side body rivets were removed off the 6464-based boxcars (done to make paint & decoration easier) and parts that were once metal were substituted with plastic (brakewheels, underframes on the reefer & lumber unloading cars, etc.)
Criticisms would also point out to the lack of big steam engines during the early years and the relatively crude Sound of Steam technology and lack of a traditional air whistle or diesel horn option in engines.
Some of the criticisms were justified as the prestige of Lionel did suffer somewhat especially with the lower-end sets (hence the commonly heard "MPC = More Plastic Crap" remarks). But the pendulum was already swinging that way even before Lionel trains was taken over by General Mills, and train sales were still in a slump for a number of years afterwards, General Mills was still experiencing a learning curve with O gauge train production and figuring out the O gauge train market. In other words, they were being cautious.
And as is true that a pendulum tends to swing back the other way, this is exactly what happened during the later part of the General Mills era when some of the higher-end engines like the FM Train Master and premium detailing gradually returning to the F3s. And as the market conditions improved, so did the variety of offerings and improvements further implemented in the product lines. Needless to say, General Mills decided to take on a product line during a time when the demand was abysmal, and kept the Lionel name alive where if given their devices, the Lionel Corporation would have surely let it continue to die a slow death.
Back to your F3s, if they look great to you, have pulling power and operational qualities that is sufficient for your needs, then there's no cause for concern.
