It's a CBT app - if it helps people feel better that's good enough for most. Perhaps a message at the beginning saying "Remember this isn't a replacement for licenced therapy so if you're feeling particularly bad contact your GP or <suicide hotline>" could be useful.

But for those who haven't got access to anything, something is better than nothing, and there is evidence even journals tracking your feelings help (which this appears to be, albeit in a more advanced form).