I tried to avoid this whole discussion altogether because the problem is that we are talking about 30 -35 year old radios. My guess is that very few, if any remaining actually continue to perform to original specifications. I have some radios whose reception seem terrific yet another example of the same model performed subpar at best. Therefore, unless we are talking about machines that were retuned, aligned, and repaired to operate as originally released when manufactured, this discussion is rather meaningless.
Below, I will show a few tuners from boomboxes that I have worked on. Let me just say that of all the boomboxes I opened up, only 2 had metal ganged variable capacitors. The rest were ALL small plastic boxes with plastic vanes. I can't speak to what it means in terms of actual performance as I've never quantified the actual performance levels, but I can say that both of the boxes that have the metal ganged varicaps were top performers. This is not to say that metal variable caps necessarily always work better. There is no question, however, that large metal ones are far and away much more expensive in manufacture and I do believe however that in designing the tuners, the choice to use significantly higher quality and expensive tuning caps probably also translate into an overall better design and better quality component selection than a tuner where a focus is on economy. When populating a circuit board with components, it wouldn't make sense to use 1 top quality component and surround it with cheap parts.
Traditionally, tuning caps in older equipment were all constructed this way. The advantage of course is durability, simplicity and ease of maintenance. If it should get dusty, it can be blown out with compressed air. If tuner cleaner is used, you can of course blow it dry. One of the main things about plastic ones is that it's semi-sealed. That should keep dust out but the advent of deoxit DIY'ers and the location of most tuners make it a price location for contamination from overuse of deoxit. Contrary to popular belief, deoxit does not evaporate residue free and will leave a film. Any kind of coating or film residue on the vari-cap will change it's capacitance and throw the alignment off -- frequently far off enough to be make it unusable. A sure fire way to temporarily disable the tuner is to spray deoxit inside of it. Don't believe me? Try it. Then turn on your tuner and see if it will receive. I bet that it will no longer receive anything except white noise. Eventually, as the solvent evaporates, it will slowly drift back to a usable range. But any film that remains may permanently alter it's capacitance. Since it is not open, moisture that enters the cap will be difficult to remove. Punch a small hole into a 2-liter coke bottle with 3 drops of water and see how long it takes for the water to completely leave the bottle -- takes a long time, doesn't it? Finally, the plastic varicap vanes are frequently warped which make it less stable in it's ability to reproduce a certain capacitance value. If you are wondering just how much of an effect a foreign object has when contacting the capacitor, try this: With the tuner working and tuned to a station, lightly touch the tuning cap with your bare fingers. At best, the tuner will immediately drift off center or to another station. At worst, it will kill the tuners ability to receive until you remove your finger.
SANYO M9990 TUNER (note plastic box with thin plastic vanes that can, and frequently do warp)
NATIONAL PANASONIC RX-7000 TUNER
JVC RC-M90 tuner. Many of you will be familiar with this tuner. And yes, it does use the plastic varicap.
JVC DC-7 The tuning cap almost looks like it belongs in a home stereo receiver. Well, the DC-7 is probably not the most portable system, so while it may technically be a boombox since it has a handle and works on batteries, I think that it's probably most at home in a den or office anyhow. This is one of the models I spoke of with an all metal tuning cap.
NATIONAL PANASONIC RS-4360dft. The tuning cap on this is very large and I've really only seen these in high quality tuner separates. This boombox was a very expensive one back in the day. $138,000 yen back in 1982. Even with ZERO adjustment for inflation, that still translate today to over $1,640 today. Adjusted for inflation using COLA figures, this translates to $3,598.10. With that type of pricing, you would expect ONLY top quality components to be selected.
(WX1) gets stations clearly... indoors with the antenna down,
. But i know there are some with tape deck. 
