They’re not the real deal. The cars may look and drive exactly as they should, but when you change the view in your garage or view them under your favorites list in arcade mode, you’ll notice they still display the name of whatever donor car you used in the white text box. Like this:
This means internally it’s still whatever donor you used, just with all its parts swapped. Pretty much, it’s a fake and if it was real, it’d have the car's real name instead. (I don't have a picture on hand at the moment) But swapping parts won’t achieve this because it again will still be whatever donor car you used. The name changer in GE 1.3.1 doesn’t change what’s in the white text box either.
It takes longer. Simple as that, it takes more time to swap parts over than it would just to activate the DLC. To be fair though, there have been shortcuts found that make this task a bit quicker, but it still takes more time to complete, especially if you don’t already have the required donor car in your garage.
You’ll most likely lose them if you update to 2.12 and beyond. So GT5 came out with update 2.12 to prevent people from modifying their save data to make their cars do something that’s otherwise impossible since it had become a huge occurrence around that time and they made several other updates after that did this. That said, if you did these part-swapping guides, theirs a good chance when you view the car, it will tell you to install the DLC (that you don’t have) or it might reset the car to whatever donor you used before. To be fair though, I have had a few that didn’t do this, but it’s weird about which ones it does this too and what it doesn’t. Still, you have a likelihood of losing them if you update to 2.12 and beyond, and even if 2.12 doesn’t reset them, later updates probably will.
These guides can only be done on 2.11. Because of what I said about updates 2.12 and later, you have to be on 2.11 to do this. This is bad if you are on any updates beyond that and have stuff such as course maker tracks in your game data (which you can’t easily back up) you don’t want to lose and don’t want to delete it just to go back. Not to mention, you’d have to wait a while for all those updates to reinstall which can be quite a pain,
especially if you have slow internet. Going through all the trouble to downgrade to 2.11 and lose whatever personal game data you have just to get these fake DLC cars is just not worth it.