Ease of backup too. One of the issues I take with browsers allowing web apps to save data locally is that they don’t also expose a way to easily locate the data in the filesystem or export it, and to my knowledge on-disk format isn’t standardized and so portability between browsers is also a concern.

The effect of this is that unless the user uses migration assistants or similar when upgrading to a new computer is that they’ll lose all local web app data. To me to this feels like a serious problem but when it’s brought up it tends to be hand waved away.