Guarantee of data backed up and restored with iTunes?

Please note that I am not asking "what" gets backed up to iTunes as there seems to be details about what is included in the back up to iTunes (or to iCloud), and I have read them.

I just wanted to know if iTunes do really back up all user data created using apps and will restore them? Say as an example, if you downloaded some journal apps and love to type up your day in the journal, or if you downloaded drawing/painting app and love creating artwork, you download a "Get Things Done" or "To-Do" app, and have jotted reminders for yourself, or you prefer mind mapping apps, and you created your mind maps.

When backing up to iTunes. Do it really back up all those files, or do it sometimes missed a file or two because of the way the app was written? Say you have a journal/diary app, do all the data you typed up in it gets backed up or do it not get backed up?

What about for example say Noteshelf app, do your books in the app gets backed up?

I just wonder because sometimes in iTunes, when your iPad/iPod is connected to computer, in the apps tab, there is a file sharing section, and sometimes some webpages suggest you copy the data to a folder for back up, but what would be the point of this if iTunes would back up the data?

When you reset your iDevice to factory setting and sync the apps back to the iDevice, then restore the back up, are you going to open any of the apps and find your work there? Or would most of it do, but a few apps are found to be empty?

What about if the app you have is no longer on iTunes Store. I meant that you do have the downloaded app on your iTunes library and can reinstall it to your iPad or whatever, but the fact that few years later, the app is not in Store could imply the developer just left, and you find that there is no way to contact the developer for help.

It's just that I'm wondering if all your user data do really get backed up or had iTunes been known to not back up this or that data?

I'm not worried about photos or videos or similar, I have copies on computer and can re-install them, I meant the work you did in your apps.

Many thanks.