There is no guarantee that the data will be persisted permanently. Users can erase it by mistake easily using privacy settings. There's also quite ambivalent size restrictions. And last but not least, Incognito mode, which also is implemented in a number of different ways in practice, depending on the browser.
Basically, you cannot be sure that you can use it to persist data at all.
Size restriction will cause error at write time, it won't silently delete data. User error is user error, that's it. I know user who was deleting files in his Windows directory to free some space. Incognito mode is not intended for web apps usage, it's more for porn and things like that, I don't think that it's very relevant.
It works for majority of standard cases and when it does not work, user will receive error message, so he'll be aware. Not the case for Apple devices anymore.
Basically, you cannot be sure that you can use it to persist data at all.