What size are the carboys? Are they the Better Bottle brand?
Assuming you are buying Better Bottles, here's what I have found:
They are fully food grade and will NOT affect the taste of the wine.
Plastic carboys don't really let in air, as most people think. If they did, that could be seen as an advantage, since much of the "concentration effect" in a neutral oak barrel is letting the wine's water and alcohol evaporate, while letting minute amounts of oxygen in.
Plastic carboys are not easily broken.
Plastic carboys cannot easily be readily moved with an air lock in place. Lifting the bottle will suck the air lock solution into the bottle. Then, when you set it down, any solution let in the air lock will be blown out of the air lock. When you get ready to move one, remove the air lock, stop up the opening in the bung, move it, then reinstall the air lock.
Six-gallon Better Bottles have a much bigger mouth on them, which makes it easier to put things like oak into them, and to get it back out later.
Because the mouth is bigger, you need a different size bung/stopper for a Better Bottle.
You can't draw a vacuum on a plastic bottle. It will simply cave the sides in until the liquid spills out the top.
That's my list, none of which is a serious negative. I like to use a Better Bottle as the fermentation's second stage carboy. The larger mouth makes it easier to transfer the oak to the carboy.
If I had a chance to get a few at a good price, I, personally, would buy them.