The problem is the level of scrutiny. Against some attackers, decoys have very very nasty game-theoretic failure cases.
Specifically- there's no limit to the number of decoys that could be on a disk. So you can get into the situation where you've decrypted every volume that exists, under coercion, but your adversary believes there are more volumes remaining.
By design, you have no way to prove that there isn't more hidden data on that disk. This is unlikely to end very well for you.
> So you can get into the situation where you've decrypted every volume that exists, under coercion, but your adversary believes there are more volumes remaining.
This is intentional.
If you could prove that there wasn't more hidden data, then the incentives would be to torture you until you did that.
Since you can't, there is no incentive to reveal a further hidden volume, since the attackers will either keep torturing you or not, and revealing more will most likely not help you.
If I remember correctly, the decoy volume treats all the hidden space as available disk space. TrueCrypt used to have a warning that booting into the decoy volume could scramble the hidden volume when the OS wrote files to disk if it happened to choose some space that overlapped with your data.
If your decoy only lists 5GB of space on a 5TB drive, then it isn't a very good decoy.
If you have a VeraCrypt partition that they can detect, it makes you look like a spy. Lots of random data in unused sectors on your hard drive is a bad look if you're trying to convince the border agent you're not a spy.
If you have a plain old laptop with some mildly embarrassing information on it that's not encrypted, you might still be a spy, but they wouldn't be able to tell from the laptop itself.
Specifically- there's no limit to the number of decoys that could be on a disk. So you can get into the situation where you've decrypted every volume that exists, under coercion, but your adversary believes there are more volumes remaining.
By design, you have no way to prove that there isn't more hidden data on that disk. This is unlikely to end very well for you.