The elements are provided on the wiki page on Hidden volume. The outer volume and the hidden volume share the same lower level space, and at creation time, the size of the hidden volume was computed to not overwrite existing data in the normal volume.
But to allow the plausible deniability, nothing in the outer volume can be aware of the existence of a hidden volume when it is opened normally (*). So if you add a lot of data into it, you are likely to overwrite the hidden volume and destroy all of its data.
Said differently, when confidentiality is important, a hidden volume is fine, but not when availability or integrity is the major concern. But even when you know that, using a hidden volume is not a trivial operation. Because you must:
- use the outer volume on a regular bases to avoid to show that it is never used => which would be an hint that is is only there to contain a hidden volume and would break in piece the plausible deniability
- never let the system file grow in the outer volume in order not to destroy the inner one (*)
(*) As it would be complex to use the outer volume and at the same time be sure not to damage the hidden one, VeraCrypt offer the possibility to make the outer volume aware that it contains a hidden one through a mount option requiring both passwords. When you use the outer volume that way, you cannot erase the hidden one. But someone looking at your computer at that moment will also know that it contains a hidden volume.
So you must be prepared if you have to reveal the outer volume to only open it in normal mode and use it normally even if it can destroy the hidden volume.