I think the Falcon is supposed to be around 50 to 100 years old as of the battle of Yavin, and it's not so much that it's a flying pile of junk/scrap, as that it's a perfect example of a jury rigged mess.
Look at it this way, the basic hull is 2 or more destroyed (totaled) ships (cars) patched togather to get something that works, then it has a drive out of something never intended to be used in that type of ship (cramming a big block v8 into a 4 cylinder motor vehicle) borderline capital ship guns (they are commonly used on a lancer frigate as anti starfighter defences) much more powerful shields than are stock, or even legal for freighters of the Falcons class, and on and on, in addition it has multiple Droid brains tied into and supplementing the main computer, and other makeshift, and jury rigged repairs to keep it running.
The ghost on the other hand is effectively a stock / professionally modified ship.